Glasses and the historical research
Jaroslav Sestak DISTINCTIVE ANNIVERSARIES, PAPERS AND CITATION RECORDS IN THE TOPIC OF GLASS CRYSTALLIZATION Sklář a keramik 11–12 / 2011 – 265
F R E D E R I K WILIE. H. Z A C H A R I A S E N BIBLIOGRAPHY
IN MEMORIAM: WALTER KAUZMANN (1916–2009)
C.A. Angell, D.R.McFarlane and M. Oguni KAUZMANN PARADOX, METASBLE LIQUIDS AND IDEAL GLASSES Annals of N.Y. Academy of Sciences
David Turnbull UNDER WHAT CONDITIONS CAN A GLASS BE FORMED? Contemp. Phys., 1969, VOL. 10, NO. 5, 473-488
Jaroslav Šesták GLASSES AND ITS HISTORY Assay
Některá výročí, publikace a citovanost v oboru krystalizace skel
Distinctive anniversaries, papers and citation records in the topic of glass crystallization
Jaroslav Šesták
Nové technologie – Výzkumné centrum západočeského regionu, Západočeská universita, Universitní 8, CZ-30114 Plzeň, E-mail: [email protected]; Sekce fyziky pevných látek, Fyzikální ústav AV ČR, v.v.i., Cukrovarnická 10, CZ-16200 Praha; E-mail: [email protected]
Práce zveřejňuje osmnáct většinou neznámých fotografií vý- Eighteen unfamiliar photos of distinguished specialists in glass značných odborníků v oblasti studia fázových transformací skel research are exposed describing also their scientific contribu- a ukazuje jejich badatelský význam. Je též otištěna fotografie tions. The work of CT7 (of ICG-International Commission on pracovní skupiny CT7 (ICG-International Commission on Glass) Glass) is mentioned including its photo from the year 2000.. dlouhodobě pracující v oblasti krystalizace skel. Text zahrnuje Text shows the survey of recently published books and reveals přehled nedávno publikovaných knih a analyzuje citační odezvy the best cited papers published in selected journals dealing with nejcitovanější publikací (podle WOS), zejména v časopisech Czech the topic of glass crystallization such as Czech J Phys, J Thermal J Phys, J Thermal Anal, Thermochim Acta, J Nocryst Sol, Phys Anal, Thermochim Acta, J Nonocryst Solids, Phys Chem Glasses, Chem Glasses, J Amer Cer Soc a Silikáty. Uvedeno přes sto citací J Amer Cer Soc and Silikáty. Text contains over hundred useful s názvy článků citations with titles.
Úvod Frederik F. H. Zachariasen (1906–1979) rakterizace skel, mezinárodně uznáva- Existuje řada významných osobností, zvažoval principy propojování atomů ný autor knih [8]. Prostřední řada: Sir které přichystali vědeckou cestu k lep- a navrhl podmínky pro koordinaci Nevill F. Mott (1905–1996) čelný autor šímu pochopení podstaty nekrystalic- nejbližších sousedů bez nutnosti krys- článků o elektronické struktuře mag- kých látek, zejména v oblasti tradičních talického uspořádání na dlouhou vzdá- netických neuspořádaných systémů, skel [1]. Především se jednalo o po- lenost [4]. Walter Kauzman (1916–2009) zejména amorfních polovodičů [9], po chopení specifičnosti krystalizačních analyzoval charakter skelného stavu němž se jmenuje přechod mezi ko- procesů při vytváření a devitrifikaci a vlastnosti tavenin při nižších teplo- vovým a nekovovým stavem („Mott skelného stavu. Historické aspekty se tách a dospěl k závěru, že entropie pod- transition“), a který je i nositelem No- zpravidla vytratily v prodloužené mi- chlazené taveniny prudce klesá během belovy ceny za fyziku v roce 1977. Jan nulosti vědeckého bádání a v následují- chlazení přes oblast tzv. skelné trans- Tauc (1922–2010) předložil nový kon- cím obr. 1 se pokoušíme připomenout formace (Tg) až v extrapolaci dosahuje cept pohledu na amorfní polovodiče, některé z nich, i když ve velice omeze- hodnoty odpovídajícího krystalické- když si uvědomil, že zakázané pásmo ném pohledu ohraničeným dostupnos- ho stavu při tzv. Kauzmanovy teplotě v pásové struktuře pevných látek může [5] tí portrétů významných osobností, což (TK) . Jakov I. Frenkel (1894–1952) se bez ostrých okrajů existovat i v neu- tak zvýrazňuje potřebu uchránit v naší zapsal do historie fundamentální prací spořádaném systému [10]. Arnošt Hrubý paměti příběhy z historie sklářského o kinetické teorii kapalin, kde skutko- (1919–) dnes často zapomínaný vynika- výzkumu, která je velkolepá, stejně vě zavedl koncept neuspořádanosti [6]. jící technolog, který uměl syntetizovat jako jeho slavní protagonisté. David Turnbull (1915–2007) se zasloužil nejrůznější amorfní polovodiče (typu o pochopení procesů nukleace a struk- III-V, II-IV) a který stanovil uznávaný Obr. 1., Některé z výrazných osobností turních transformací a zavedl koncept (Hrubého) koeficient sklotvornosti [11]. nauky o skle, horní řada z leva: Gus- redukovaných teplot (např. Tg/Tm = Tgr) Hywel A. Davies (1941–) emeritus, Sheffi- tav H. J. Tammann (1861–1938) který čímž upozornil, že i sklotvorné proce- eld univerzita, vylepšil výzkum a tech- jako první podtrhnout důležitost jevu, sy se mohou popsat kvantitativně [7]. nologii přípravy kovových skel ultra- že čím je vyšší viskozita taveniny při Miloš B. Volf (1915–1983) dnes bohu- rychlým chlazením, připravil amorfní teplotě tání tím nižší je její tendence žel opomíjená individualita českého a nanokrystalické magnetika. Donald ke krystalizaci [2] stejně tak jako prv- sklářství, významný svými encyklope- R. Uhlmann (1935–) emeritus, dřívější ní zavedl termín termická analysa [3]. dickými schopnostmi nejrůznější cha- ředitel význačného Výzkumného cent-
264 – Studie Obr. 1 – Některé z výrazných osobností nauky o skle ra Arizonské university (Tucson), uzná- Alfred Univerzity, studoval krystali- časopisu, který se počítá v průměru po- vaným spoluautorem Kingeriho knihy zační procesy [21], vitrifikace nukleární- sledních dvou let) [46] nedílnou součástí o základech nauky o skle a keramice ho odpadu a podporoval mezinárodní hodnocení vědeckých výstupů odborné a blízkým spolupracovníkem Norberta spolupráci (vznik a editace nového časo- práce širokého okruhu badatelů [46–48]. J. Kreidla [13, 14]. Edgar D. Zanotto (1954–) pisu Journal of Applied Glass Science). Nejpoužívanějším přístupem je využi- São Carlos univerzita (Brazilie) se za- Cornelius T. Moyniham (1937–) emeritus, tí databáze WOS (= Web of Science [48], sloužil o porozumění procesů nukleace Rensselaer polytechnika (Troy) zavedl který je použit i v tomto článku) a krystalizace skel [15,16] a je význam- koncept fiktivní teploty pro charakteri- V letošním roce se sešlo několik důle- ným editorem žurnálu nekrystalických zaci struktivní relaxace skel [22]. žitých výročí renomovaných autorů látek. Spodní řada: Hiroshi Suga (1930–) Na kontinuálním pokroku nauky v oblasti studia skelného stavu. Před emeritus, Osaka univerzita (Japonsko) o skelných přeměnách se významně po- čtyřiceti lety na přelomu sedmdesá- zakladatel teorie a klasifikace nekrys- dílela i speciální skupina „C7 = Nukleace tých let byl vědecký výzkum tehdejší- talických látek včetně originálních a krystalizace skel“, která byla ustavená ho Ústavu fyziky pevných látek (ÚFPL studia kalorického nízkoteplotního před 30. lety v rámci mezinárodní spo- ČSAV, předchůdce dnešního Fyzikál- chování specifického tepla [17]. Bernhard lečnosti skel (ICG) a jejíž historické foto ního ústavu, FzÚ AV ČR) zaměřen na Wunderlich (1931–) emeritus, Tennessee je ukázáno v obr. 2. Neměli bychom ani zkoumání polovodivých vlastností nej- univerzita (Knoxville), zajistil klasifi- opominout českou stopu zejména v ob- různějších materiálů. Jeho pracovníci kaci stupně krystalinity v makromo- lasti knižní propagace nauky o sklech tehdy znali jak připravit vysoce čisté lekulárních látkách, zabýval se struk- [8, 10, 23–33] ani mezinárodní knižní pub- germanium a křemík, tak i možností turou látek existujících mezi kapalným likace z posledních deseti let [32–45]. jejích dopování (vnesení příměsí) a sou- a pevným stavem [18]. Charles A. Angell časně dospěli v oblasti studia, přípra- (1933–) emeritus, Arizona univerzity vy a chování amorfních chalkogenidů (Tampe) se zabýval studiem struktury Citovanost publikací k poznání, že se jedná o významnou kapalin a jejími transportními a ter- a některá významná výročí inovaci v oboru skel. Tzv. zakázaný modynamickými charakteristikami v oboru nauky o sklech elektronový pás zde existuje i přes ab- a upřesnil metastabilní stavy podchla- senci dlouho-dosahového krystalového zení a fragilitu skel [19]. Larry L. Hanch V současné době se stal citační rejstřík uspořádání, který není ani v nekrys- (1937–) emeritus, Florida univerzity, autorů (tzv. citovanost reprezentující talickém blízko-dosahovém spořádání který se zasloužil o pochopení bio- četnost odkazů na jednotlivé publika- prázdný, protože obsahuje dostatečné aktivity anorganických povrchů skel ce) stejně jako tzv. impaktní faktor od- množství lokalizovaných stavů, jen a jejich možnou implantaci do živých borných časopisů (≈IF, tj. citační odkazy vykazuje neostré okraje. Spoluobjevite- tkání [20]. David L. Pye (1937–) emeritus, vlastních článků na stránkách daného lem fundamentálního pojednání o op-
Sklář a keramik 11–12 / 2011 – 265 tické absorpční hraně v tetraedrických základě experimentálně stanovených přilákala nebývalé množství interpreta- polovodičích byl výše zmíněný J. Tauc, charakteristických teplot získaných cí [59, 60] a lze ji tak srovnat s jinou světo- jeden z nejvýznačnějších českých fyzi- pomocí metody diferenční termické vě uznávanou rovnicí nazývanou „Jan- ků posledních let. Jeho článek v Physics. analysy (DTA) studia chalkogenidů, der diffusion equation“ [61] (s 550 citacemi, Status Solidi (IF = 1.15) z roku 1966 [49] dodnes nazývanou „Hrubý glassforming IF = 1,23). Předchůdcem tohoto článku získal chvályhodných 1 375 citačních coefficient“. Tento jeho článek, publi- v TCA bylo pojednání J. Šestáka (1938–) ohlasů, čímž se stal nejlépe citovaných kovaný v Československém časopisu pro o kinetice neizotermních procesů pu- článků tohoto časopis. Za zmínku sto- fyziku (IF = 0.57), získal 372 citací a stal blikovaném v českém časopise Silikáty jí, že nedávno zemřelý J. Tauc po roce se nejlépe citovanou publikací tohoto (IF = 0,66) už v roce 1967 [62], který opět 1968 emigroval do USA (Brown Uni- časopisu (s téměř dvojnásobnou citova- dosáhl největší citovatelnosti v rámci versity, Rhode Island) kde dále zkou- ností než následující článek) a to v celé celé historie tohoto časopisu‚ nyní Ce- mal a odhaloval podstatu objemového jeho historii, který bohužel v důsledku ramics-Silikáty) s 61 citačními odkazy. fotovoltaického úkazu, anomálního finančních potíží přestal v roce 2006 Širokou citovanost kinetických teorií termoefektu, fotomagnetického a fo- existovat. Tento Hrubý koeficient byl krystalizace poskytuje časopis Journal topiezoelektrického jevu což shrnuje v následujících letech mnohokrát kriti- of Thermal Analysis and Calorimertry ve své knize o amorfních a kapalných zován a matematicky upravován [54, 55], (JTAC, IF = 1.45) [48], zejména reprezento- polovodičích, publikované nakladatel- nicméně nedávná analýza poukázala vaný pracemi T. Ozawy (1932–), např. [63] stvím Pergamon Press v roce 1974 [10]. na fakt [56], že jeho původní prospěš- s 1 053 citacemi, který tak předstihl ná- Rozsah citací Tauce je srovnatelný s nej- nost a užitečnost přetrvala všechny sledující článek instrumentálního cha- lépe citovaným článkem časopisu Jour- následné modifikace. rakteru [64] o plných 600 citací. Aplika- nal of Noncrystalline Solids (JNCS, IF = Hledání vhodného popisu a charakteri- cemi DTA v oblasti krystalizace skel se 1,44) z pera dalšího polovodičového zace krystalizačních pochodů ve sklech zabývá řada časopisů specializovaných velikána N. F. Motta o povaze vodivosti vedl v tehdejším ÚFPL i k dalšímu od- na studium skel, např. TCA [65], PCG [12, skel [9] s 1396 citacemi, který tak pře- bornému pojednání tentokrát publiko- 66–68] s 201, 291, 243, 199 a 155 citace- stihl druhý nejcitovanější článek JNCS vanému v časopise Thermochimica Acta mi což je srovnatelné s JNCS [69–71] s 288, o struktuře chalkogenidů [50] o plnou (TCA, IF = 1.75) a dodnes citovanému jako 417 a 445 citacemi. Viskozitní měření polovinu. Tento výsledek je srovnatelný „Šesták-Berggren kinetic equation“ [57] (na- přilákalo vždy hodně citačních odkazů, s termofysikálním měřením vodivosti víc zajímavém tím, že oba autoři se spo- příkladem je časopis Journal of American skel, publikovanými v časopise Physics lupodíleli i na založení tohoto časopisu Ceramic Society (IF = 1.94) s historickým and Chemistry of Glasses (PCG, IF = 0.58) nakladatelstvím Elsevier v roce 1970). rekordem 1 913 citací článku [72] z pera [51,52] s 769 a 581 citacemi Tento článek získal 562 citačních odkazů G. S. Fulchera (1884–1959) sledovna- Taucovým tehdejším spolupracovníkem a tím se stal nejlépe citovaným článkem ný materiálově zaměřeným článkem v ÚFPL (viz historie [53]) byl A. Hrubý, v celé historii tohoto TCA časopisu (a to L. L. Henche [20] s 1 798 citacem a nebo který syntetizoval a zanalyzoval stovky v rozsahu jeho téměř 22 tisíc dosud pub- studiemi C. A. Angela [19] a B. Wunderli- světově originálních chalkogenidových likovaných článků), čímž předstihl cito- cha [18] s téměř tisícovkou citací (tito vý- (zejména amorfních) sloučenin, a kte- vanost následující publikace a dynamic- znační autoři vykazují nejen extrémně rý v roce 1972 publikoval pojednání ké měření velikosti pórů [58] o vice než vysokou celoživotní citovanost s ~15 000 popisující sklotvornou tendenci [11] na 200 citací. Takto zobecněná SB rovnice odkazy, ale také vrcholný tzv. H-index (> 60), ukazující jejich schopnost vytvá- řet navazující tematicky-orientované skupiny-školy [48]). Nemenší pozornost si zaslouží články zabývající se specifikou měření a interpretace teploty anotované např. C. T. Moynihanem [22, 73] s 609 a 109 nebo D. Turnbulla [74] s 695 citacemi. Pro zajímavost jeden z nejcitovaněj- ších článků o krystalizaci je publikace D. R. Uhlmanna v JNCS [70] zabývající se neizotermní krystalizaci skel, která je však v úvodní části (pojednávající o stavových rovnicích kinetiky) chybná [75] což navozuje otázku co je vlastně důvodem největší citovatelnosti člán- ků [48], jejich hluboké odborná úroveň nebo popularizační schopnost komuni- kace a propagace. V tomto schématu samozřejmě chybí ne- Obr. 2 – Pracovní skupina ICG-TC7,která na přelomu třetího tisíciletí pracovala ve složení, zprava G. Völksch (Germany), V.M. Fokin (Russia), M. Davis (USA), R. Müller (Německo), ze- dávno založené časopisy, jako je Intern- snulý P. James (UK), v podřepu E. Zanotto (současný předseda,, Brazilie), zesnulý M. Weinberg tional Journal of Applied Glass Science (USA), W. Hölland (předchozí předseda, Liechtenstein), T. Kokubo (Japonsko), zesnulý I. Szabo
266 – Studie i Česko-Slovenští členové jeho redakční přetržitém vývoji jak ukazují i cíleně nidových [79, 87] a fosfátových [88] skel, rady A. Helebrant, M. Liška a J. Šesták) publikovaná a speciálně zaměřená rozvoj oblastí magnetických vlastností stejně jako časopisy, které IF dosud bo- kompendia [32, 33, 80–83]. Je potěšující, [89], bioaktivity [90], modelování [91], skel hužel nemají (jako Sklář a keramik). Zá- že i Česko-Slovenští autoři se prosadili s nukleárním odpadem [92], tavených věrem si připomeňme, že nesmíme za- v celosvětovém badatelském zápolení čedičových vláken [25, 93] nebo polysia- pomenout na velikány českého sklářství o rozsáhlejší citovanost [48] a přispěli látových geopolymérů [94]. Můžeme se [1, 53], z nichž někteří jsou autory výše tak k rozvoji uznávaného oboru chová- i pochlubit i nedávno publikovaným zmíněných knih [38–43]. ní nekrystalických, amorfních či skel- článkem v renomovaném časopise Na- Specifická oblast studia krystaliza- ných látek [53, 84], kde stojí za zmínku ture (IF = 34.5) [95], bohužel rozsah to- ce skel je nadále středem zvýšeného třeba kinetika krystalizace [65, 74, 85–87], hoto příspěvku neumožňuje ocitovat zájmu [18, 19, 35, 75–78] a je i v dalším ne- charakterizace kovových [86], chalkoge- veškeré československé prameny [53].
Poděkování: Tato práce byla realizovaná s podporou projektu CENTEM, reg. no. CZ.1.05/2.1.00/03.0088, který je spolufinancován ERDF v rámci program OP RDI Ministerstva školství, mládeže a sportu. Autor děkuje za spolupráci prof. Marku Liškovi a PhDr. Petru Novému.
Lektor: prof. Ing. Marek Liška, DrSc.
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Sklář a keramik 11–12 / 2011 – 269 national academy of sciences
F r e d e r i k w . h Z a c h a r i a s e n
1906—1979
A Biographical Memoir by mark g. ingraham
Any opinions expressed in this memoir are those of the author(s) and do not necessarily reflect the views of the National Academy of Sciences.
Biographical Memoir
Copyright 1992 national academy of sciences washington d.c.
FREDERIK WILLIAM HOLDER ZACHARIASEN
February 5, 1906-December 24, 1979
BY MARK G. INGHRAM
REDERIK WILLIAM HOLDER ZACHARIASEN'S contributions to Fscience have been rich and varied. He was a leader in the determination of the crystal structure of inorganic crystals using x-ray diffraction. Though primarily an experimen- talist, he contributed to theory whenever he found the theory inadequate. In over 200 published papers he included ex- periments on the crystal structure of minerals, on the structure of inorganic crystals, on the structure of anionic groups, on atomic and ionic radii, on the glassy state, on the liquid state, on actinide crystal chemistry, on high-pressure phases, on crystal structure and superconductivity, on the melting process, and on the variation of bond strengths with bond lengths. His contributions to theory include papers on tem- perature diffuse scattering of x-rays, on stacking disorder, on the phase problem, and on extinction including the Borrmann effect. Each of these theoretical efforts was fol- lowed by careful experimental investigations to establish the correctness of the theory he had developed. Linus C. Pauling of the University of California at Berke- ley, who also concentrated on the determination of crystal structure during his early years, said of Zachariasen's work (1975), "I feel that he is to be classed among the outstand- 517 518 BIOGRAPHICAL MEMOIRS ing scientists of the twentieth century, and at the top in the field of inorganic crystal structures." Robert Penneman of the Los Alamos Scientific Laboratory said (1975), "The breadth of his contributions is enormous; there is no ma- jor advance in crystalography in one half century that does not bear his mark." Bernd T. Matthias of the University of California, San Diego, said (1975), "His was a monumental achievement in understanding the detailed nature of the whole periodic system." Such accolades abound.
THE MAN Zachariasen had absolutely no use for pretense or titles. His friends and associates always called him by one of his two nicknames, "Willie" or "Zach." It would give completely the wrong impression of "The Man" if I were to refer to him as "Zachariasen" in this memoir. I will therefore use the name he preferred: "Willie." He would want it no other way. Willie was born in Langesund, at the mouth of the Langesundfjord, in southeastern Norway. Willie's father was a sea captain. Langesund is about 15 kilometers from Brevick, which is at the center of the nepheline-synetic pegmatite veins which have yielded over thirty new species of minerals. The islands of the Langesundfjord are also rich in deposits of rare silicates and other well-crystallized minerals. Raymond Pepinsky, one of Willie's early Ph.D. students, relates a story in which Willie, decades after his youth, correctly identified on sight a Langesund eudidymite specimen which had been mislabeled by the Krantz firm in Germany. X-ray examination proved Willie correct. "Willie, how could you have known?" he was asked. "I played on the Langesund islands when I was a boy," he replied with his usual warm grin, "and I remember those crystals." Such mineral crystals so interested Willie that when he went to FREDERIK WILLIAM HOLDER ZACHARIASEN 519 Oslo University in 1923, he studied in the Mineralogical Institute under the guidance of the great geochemist Victor Moritz Goldschmidt. Goldschmidt (1888-1947) was one of the first to recog- nize that crystal structure data as dietermiiaed using x-rays could reveal the distribution of elements within crystalline minerals. Among the crystals which; interested Goldschmidt, and whick Willie studied while still in Oslo, were some rare earth crystals. Crystals containing these elements abound on the Langesundfjord islands. As Pepinsky tells us, "In order to protect the richest of such deposits for his own leisurely study of its mineral species, Goldsehmidt purchased one of the islands Willie knew best." Willie published his first paper, "IJber die Kristallstrucktur von BeO," in 1925 at the age of nineteen. He published nineteen more papers before publishing his Ph.D. thesis in 1928 at the age of twenty-two. He was awarded the Ph.D. that same year. He was then appointed assistant professor at the University of Oslo, but was granted a leave for 1928- 29 to accept a Rockefeller Foundation Fellowship to study with Sir Lawrence Bragg at Manchester University in En- gland. After his postdoctoral at Manchester, Willie returned to Oslo. Early in 1930 Willie received an offer from Arthur Holly Compton, Nobel Prize winner for the discovery of the Compton effect, to join the faculty of the Department of Physics of the University of Chicago as an assistant pro- fessor. Willie had been recommended to Compton by Bragg. Willie accepted that appointment and stayed at the Univer- sity of Chicago until he retired in 1974. One week before Willie sailed for New York he married Ragni (Mosse) Durban-Hansen, granddaughter of the pio- neer Norwegian geochemist W. C. Br0gger (1851-1949). Br0gger, among other things, had discovered and first 520 BIOGRAPHICAL MEMOIRS described the minerals of the Langesund region. In many of Willie's publications on the structure of minerals he refers to "Br0gger's well-known monograph on the pegma- tite minerals of the Langesundfjord." Clearly Willie loved and was immersed in the study of minerals and crystallog- raphy while young and in Norway. The invitation to the University of Chicago in 1930 was an effort on Compton's part to build up x-ray studies at Chicago. It is of interest to note that, at that time, Compton was not chairman of the Department of Physics. He built up the group in x-rays almost on his own. Willie arrived on campus on the same day as another new assistant professor active in x-ray studies: Samuel King Allison. Allison had also been invited to join the faculty by Compton. Allison already knew the campus since he had grown up in the University area and had received his Ph.D. from the Uni- versity seven years earlier. The Zachs and the Allisons be- came the closest of friends—a friendship that lasted the rest of their lives. Also added to the faculty in 1930 as an instructor was Ernest O. Wollan. In addition to these four faculty members active in x-ray studies there were three postdoctorals: Marcel Schein from 1929-31 as a Rockefeller Foundation fellow, Elmar Dershem from 1929-42 as a re- search associate, and John H. Williams from 1931-33 as a National Research Council fellow. Williams had earlier worked with Allison at the University of California, Berkeley. In addition to the very close friendship with the Allisons, the Zachs also became close lifelong friends with the Scheins and John Williams. Willie had little respect for Compton as a person. Three of these seven—Allison, Compton, and Williams—worked on the physics of production of x-rays and on their interaction with matter. Dershem and Schein worked in similar areas, except with very soft x-rays. Wollan worked on the scattering of x-rays by gasses. Willie was FREDERIK WILLIAM HOLDER ZACHARIASEN 521 alone in crystal structure determination. During the few years this group was together, almost all of their research was published independently. There were three exceptions, two papers by Dershem and Schein and one by Wollan and Compton. Compton and Allison did publish jointly an ex- tensive and excellent reference book, X-Rays in Theory and Experiment (1934). In spite of the extensiveness of that book, it contained nothing on Willie's love, crystal structure. Shortly after that book was written, all except Willie changed their fields of research: Compton, Dershem, and Schein to cosmic rays; Allison, Williams, and Wollan to nuclear physics. Willie continued, single-mindedly, to determine what he could about the structure of matter using x-rays as a tool. Willie had very little financial support for his work dur- ing the thirties. Aside from funds to support Compton's work (supported in part from the outside) and operating A. A. Michelson's two grating ruling engines (maintained during that period by the chairman of the department, Henry Gordon Gale), there was very little money left for others. Willie had to use homemade x-ray tubes built by his associate Dershem. When in 1938 his kenotron recti- fier burned out, he had to shut down his research for six months until the department could find the $75 necessary to buy a new rectifier. All trips to meetings of the Ameri- can Physical Society were at participants' expense. Willie had to make his own slides. A typical procedure for such trips was for Willie and Sam Allison to take one car, add as many of their students as possible and head for the meet- ing. If the meeting was to be in Washington, D.C., for example, they would stop overnight at the Gamma Alpha house (scientific fraternity) at the University of Ohio, and then at a cheap tourist house in Washington. This lack of support from the department of which he was a member continued until 1943. 522 BIOGRAPHICAL MEMOIRS In 1943 Willie joined the Manhattan Project. During the next two years he helped to unravel the chemistry, and to determine the crystal structure, of the transuranic ele- ments and compounds. Many who were involved in that project feel that it would have taken much more time to do these jobs if it had not been for Willie's insight and genius in crystal structure determination. Late in 1945 Willie first accepted administrative duties. His influence and effectiveness in these positions has posi- tively affected many lives. In 1928, just two years before Willie went to the University of Chicago, a national survey had rated the Department of Physics of that university num- ber one in the country. This was due in large part to the presence at that time of Michelson, Milliken, and Compton, three Nobel Prize winners. During the thirties, under the guidance of Gale and Compton, that rank slipped badly. This, according to Willie, was due primarily to the auto- cratic rule within the department, and to the hiring by the department of its own students as junior faculty, largely to assist the faculty member under whom that student had received the Ph.D. degree. The changes Willie made were momentous and lasting. He immediately ended the domination of the department by Michelson's grating ruling engines by givng them away, one to Bausch and Lomb, and one to the Massachusetts Institute of Technology. He immediately turned the de- partment from autocratic to democratic. The then tenured staff of the department met for the first time in many years to consider departmental affairs. Without much delay they voted to terminate the appointments of ten nontenured staff members. They also voted to reject as a member of the Department of Physics a new professor whom Compton had just hired, as usual without consulting anyone else in the department—a ticklish situation which Willie had to FREDERIK WILLIAM HOLDER ZACHARIASEN 523 handle. A position for this professor was finally found in another part of the university. Next, the staff voted to appoint Enrico Fermi and Ed- ward Teller as professors, Robert F. Christy and Walter H. Zinn as associate professors, and John H. Simpson as in- structor. It was Willie's job to invite these men to join the staff and to persuade them to accept. Willie did invite them, and all accepted. With this success, the Chemistry Depart- ment, partly due to Willie's needling, invited Willard F. Libby, Joseph E. Mayer, and Harold Urey. This enabled Willie, with the support of the physics faculty, to invite Maria G. Mayer to become a volunteer professor of phys- ics, since the university's nepotism rules at that time for- bade two members of one family holding faculty positions. She accepted. A bit later, with the support of the physics faculty, Willie invited Gregor Wentzel and others. The fact that all these outstanding physicists accepted positions at the University of Chicago is testimony to Willie's persua- siveness and the confidence which people put in his work and leadership. With this staff there was no trouble in attracting the most oustanding students in the country. By 1949 the department was once again rated tops in the na- tion, and among the Ph.D.'s graduated between 1945 and 1950 were five who were later awarded Nobel Prizes in physics. As soon as Willie took over, with the unanimous support of the faculty, Willie introduced bylaws by which the de- partment was to operate. According to these bylaws the department was no longer to be administered by a chair- man who acted as a head, but by a true chairman. To Willie "chairman" meant that the person administering the department could do only those things that the faculty voted while the chairman occupied the chair at faculty meetings. All faculty were to vote on new appointments; 524 BIOGRAPHICAL MEMOIRS all faculty of higher rank on promotions. A policy commit- tee, a budget committee, a curriculum committee, a ser- vices committee, etc., were established made up of faculty, by vote of the faculty. With this reorganization the old autocratic procedures of Gale and Compton were gone. As far as graduate students were concerned, Willie per- suaded the faculty to accept the rule that all Ph.D. theses had to be published under the students' names alone. He felt that if a piece of research had not been done indepen- dently enough to justify publication by the student alone, it was not acceptable as establishing that the student could do independent research. Willie's standards were always very high. This rule held until the late 1960s when excep- tions were made for students in high-energy physics, where a staff member's participation was required before that stu- dent could get access to a large accelerator. Willie had a heart attack in January 1949 and a second attack four months later. With the second heart attack Willie resigned the chairmanship of the Department of Physics and slowed down a bit. He had published twenty-seven papers in 1948-49. Willie, however, was not one to walk on eggs. In 1954-55 he published fifteen papers. With the department once again going downhill, Willie was drafted once again to take over the chairmanship in 1956. Again he worked his magic. In 1959 the faculty of the Division of Physical Sciences persuaded him to take over as dean of the division. Again he did his magic, and the caliber and support of the faculty throughout the division improved. Having accomplished what he thought he could, he re- signed as dean in 1962, two years before his term was up, and returned to his research. In 1970 Willie had a bad case of phlebitis. His best friend Sam Allison had died of complications from phlebitis a few years earlier. Two heart attacks and phlebitis were enough to cause him to FREDERIK WILLIAM HOLDER ZACHARIASEN 525 rethink his future. He was just at retirement age. He was offered a special postretirement appointment but decided to accept that appointment only part time, so that he could spend some time "living." Since Willie and his wife, Mossa, had many friends from the Manhattan Project living in Los Alamos and its neighbor Santa Fe, New Mexico, they de- cided to move to Santa Fe. There they purchased the first home they had ever owned. Willie did as he had agreed; he and Mossa returned to the University of Chicago for one quarter of each year, for three years, to give one course. He kept quite active in research through his contacts with associates in the Los Alamos scientific laboratories, mainly his friends Finley H. Ellinger and Robert A. Penneman. He also worked with his friend and associate Bernd Matthias at the University of California, San Diego. He published several papers with each. He also spenttime enjoying food, music, art, mystery stories, Indian lore, his home, and "liv- ing." Willie was a superb teacher both at the graduate and undergraduate levels. I well recall one course I took from him in graduate-level classical mechanics in the late thir- ties. This course was considered one of the very best in the department at that time. Willie would enter the lec- ture room, place his notebook on the desk, and proceed to give a beautiful, understandable, and rigorous lecture. Then, after answering questions he would pick up his still-closed notebook and leave. Only once in that particular eleven- week course did he open that notebook to check on a formula he had derived. He decided that it was correct, though in a different form than his notes. He then closed his notebook and finished the lecture. He just never made mistakes. It was not until years later that I learned that such lectures were the result of careful preparation on his part. Some years later, when I was chairman of the De- 526 BIOGRAPHICAL MEMOIRS partment of Physics, I asked Willie to give an introductory course in physics (physical science) for nonscience students at the undergraduate level. He agreed and did a superb job in this difficult assignment. Student evaluations were enthusiastic. He developed a close personal relationship with the roughly 150 students in his class. The course was given during the period of student protests in the late 1960s. The University of Chicago students were no different from any other college students, and students, including stu- dents from his class, took over the administration building of the university. Willie was one of a few faculty whom the students would let into that building. They obviously en- joyed conferring with him. He helped calm troubled waters. Willie sponsored a number of students for their Ph.D. degrees, all of these in the period 1930-40. His Ph.D. stu- dents were John Albright, Donald A. Edwards, Ssu Mien Fang, Jane (Hall) Hamilton, Dorothy Heyworth, Richard C. Keen, Raymond Pepinsky, Stanley Siegel, Rose (Mooney) Slater, and G. E. Ziegler. He had two assistants while he was working for the Manhattan Project: Wallace Koehler and Ann Plettinger. Koehler stayed with Willie for a few years after World War II working toward a Ph.D., but he did not finish. Plettinger stayed with Willie as an assistant until he left Chicago. She was coauthor with Willie on nine of his post-World War II papers. Willie accepted no students seeking advanced degrees during or after World War II. He felt that the work he was doing was chemistry, not physics, and that it was not a suitable field of research for physicists. He was in a physics department. Willie did ac- cept several postdoctorals who came to him with outside named fellowships. He used the criterion of outside sup- port as one indication of the independence, competence, drive, and real interest of these people in the work he was doing. He felt that if he provided the support, he would FREDERIK WILLIAM HOLDER ZACHARIASEN 527 get people who simply wanted a job and weren't good enough to get their own support, or a faculty position elsewhere. During Willie's first twenty years at the University of Chi- cago his favorite vacation was a stay with his friend Sam Allison at Three Lakes in Wisconsin or, more frequently, a fishing-canoeing trip into the Lake of the Woods area in northern Minneosta with his friends from his early years in Chicago, Sam Allison and John Williams. Williams had gone to the Department of Physics at the University of Minne- sota. The fourth person on these trips was generally Buddy Thorness, also from the University of Minnesota. Willie loved the woods, the water, the portages, the fishing, the cooking, and most of all the repartee with these close friends. After Willie's two heart attacks in 1949 the frequency of these canoeing trips dropped off. For relaxation Willie then became a devotee of the billiard room of the faculty club of the University of Chicago for a short game after lunch. The favorite game soon became a frustrating game called "Cowboy Pool." It was the game of choice for Willie because it is a game that cannot be played by a person without a sense of humor. As his ofttimes partner in these games Julian Goldsmith has said: Willie was the leader of the group, made up of people of diverse and unrelated interests. He set the tone and developed refined rules, designed to eliminate the trivial and make the game more challenging—typical of Willie. He set the standard for gamesmanship, repartee, razzing, hexing, friendly insult, amateurism, and comradery that made winning or losing of little importance. His influence made the game a subtle Rorschach test, and the real nature of the players became quickly evident. Willie's humor and strength of personality pervaded the room. He had a rapier-like wit. He added to what may sound like a sterile activity. With Willie it wasn't.
THE SCIENCE Willie was always interested in the structure of matter that x-rays could reveal. He was not a developer of new x- 528 BIOGRAPHICAL MEMOIRS ray instrumentation or techniques. The techniques he learned while a student of Goldschmidt at Oslo University were the Laue (single crystal), the Debye-Scherer (powder), and the rotating crystal techniques, all of which used photographic recording. During his postdoctoral period with Sir Lawrence Bragg in Manchester University, Willie learned the Bragg technique of measuring the intensities of reflections from single crystals by means of ionization chambers, and the use of those measurements to derive Fourier diagrams (two- dimensional representations) of electron distributions within crystals. After World War II Willie adopted, and con- tributed to the development and use of, the single fixed crystal spectrometer using proportional counters for mea- surement of spatial intensity measurement. He did on a few occasions write papers in which he used neutron dif- fraction to determine the position of light elements within crystals, e.g., the rare earth hydroxides (1955) and MgH (1963). In these few cases Willie did the interpretation of the data, and his associates determined the neutron dif- fraction patterns. Willie was also a very good chemist. His mentor Goldschmidt had prepared a number of the compounds and crystals that Willie used while a student in Oslo. Willie later pre- pared a number of compounds and crystals for his own use. After World War II, Willie had his own small chemis- try laboratory. In that facility, among other things, he pre- pared a number of fluoride compounds and metaborates. He also grew crystals, e.g., of the metaborates, of sufficient size and perfection to do single crystal studies. Mineralogical Crystals As detailed earlier, Willie was born and grew up in Langesund, Norway, an area rich in well-crystalized min- erals. He was intrigued from the very beginning with the FREDERIK WILLIAM HOLDER ZACHARIASEN 529 structure of these minerals. His first published paper was on the crystal structure of BeO, which as a mineral is called Bromellite, named for the Swedish mineralogist who dis- covered it. His second paper was on Wurtzite (ZnS) and the related crystals a and P CdS. Most of Willie's first thirty-four papers concerned minerals or compounds of interest to mineralogists. Specifically, and in order, the minerals he studied after Wurtzite included Phenacite (Be2SiO4), Wille- mite (Zn2SiO4), Montroydite (HgO), GeO2 isolated from Argyrodite, Bixbyite ([Fe,Mn]2Os), Titanite (CaTiSiO5), Eudi- dymite and Epididymite (NaBeSi3O7[OH]), Thortveitite (Sc2Si2O7), Benitoite (BaTiSi3O9), Hambergite (Be2BO3[OH]), and Colusite ([Cu,Fe,Mo,Sn]4[S,As,Te]^4). Along with these structure determinations, Willie discussed the structure of a number of minerals having analogous structures and formulas. As a survey of these minerals shows, as time went on, Willie determined the structure of more and more complex minerals. Inorganic Crystals Willie's interest in the structure of inorganic crystals in general and the reasons for variations in those structures become apparent in his years in Oslo. He did not just study the mineral Wurtzite (ZnS), he also investigated the chemically similar crystals ZnSe and ZnTe; not just BeO but also BeS, BeSe, BeTe, and MgTe; not just CdS but also CdSe and CdTe; not just HgO but also HgS, HgSe, and HgTe. He also made systematic studies of sesquioxides (X2O3) and crystals of the form AXO3 (1928,3). Such studies enabled him to make correlations and generalizations. One important piece of work of this type was his publication of tables of atomic radii. His first publication of a paper spe- cifically on this topic was in 1931. Some of Willie's extensive general and systematic studies 530 BIOGRAPHICAL MEMOIRS of inorganic crystal structure are sufficiently distinct to be listed separately. Groups in Crystals As Willie said in the introduction to a number of his papers, "I have been interested in the determination of the shape and accurate dimensions of inorganic groups in crystals." His interest was not only in the shape and di- mensions, but variations in those parameters for the same group, in different crystals, and the reasons for those varia- tions. This interest first appears in print in his thesis where, among other things, he studied the shape of XO3 groups. The interest continued throughout his career. His first paper specifically on the subject, "The Structure of Groups XO3 in Crystals," was published in 1931. He published thirty- three more papers, which in large part were studies of groups. He distinguished groups from radicals in the sense that an XO3 group in a long string of XO3 groups in which O's are shared between all adjacent groups is a group, it is not a radical. He correlated the structure of these groups with the number of valence electrons in the group. In his 1931 paper on XO3 groups he showed that XO3 groups 1 2 having twenty-four valence electrons, e.g., (NOg)" , (CO3)" , 3 and (BO3)' , have coplanar structure, while those having 3 2 1 3 26 valence electrons, e.g., (PO3)" , (SO3)" , (CIO,,)" , (AsO3)" , 2 1 3 (ScO3)' , (BrOg)' , and (SbO3)- , are pyramidal. In a 1932 paper entitled "Note on a Relation Between the Atomic Ar- rangement in Certain Compounds, Groups and Molecules and the Number of Valence Electrons," he refined these ideas. He made predictions based on valency for XO2 groups 1 and, for example, predicted that the (NOg)" group in NaNO2 would not be linear. He put his student G. E. Ziegler to the task of checking this prediction. It proved correct. Willie also pointed out that crystals whose chemical formulas are AXO3 FREDERIK WILLIAM HOLDER ZACHARIASEN 531 do not necessarily have XO3 groups. He demonstrated this to be the case in LiIO3, NaIO3, and CsIO3, where iodine oc- curs in IO3 octahedra which share corners with one another. In another series of systematic investigations Willie stud- ied the structure of SnOm groups. Specifically, he deter- 2 2 mined the structure of sulphite (SO3)" , pyrosulphite (S2O5)~ , 2 2 persulphate (S2O8)~ , and trithionate (S3O6)~ during the 2 period 1931-34. The structure of sulphate (SO4)" had been determined earlier by others, but controversy still existed about its structure in Na2SO4 and AgSO4. Willie redetermined 2 these structures. The dithionate group (S2O6)" had been de- termined by others earlier. Based on such studies Willie showed that the pyrosulphite group should be written as SO3SO2 linked by a covalent bond between the two sulphurs, not as SO2SSO2, as had been previously assumed. Willie also showed that the persulphate group could be described as two SO4 groups linked together by a covalent bond between two of the oxygen atoms SO3OOSO3 and that the trithionate group could be described as two SO3 groups bonded to a com- mon sulphur atom SO3SSO3. Such generalizations had ob- vious implications for later investigators who investigated other compounds containing these groups.
Borate Groups The most extensive series of investigations of groups that Willie undertook was the determination of the structure of borate groups and of borates. This series of investigations continued through sixteen papers extending over thirty- three years. He introduced some of these papers in the later part of the series with the phrase, "This investigation is part of a systematic study of borate structures being car- ried on in this laboratory." As one might guess from Willie's history, the first borate Willie investigated was of a min- eral, Hambergite, Be2(BO3) (OH) (1931,1), in which he found 532 BIOGRAPHICAL MEMOIRS the borate group to exist as an almost perfect O3 triangle with the boron atom at its center. He reexamined this crystal in more detail in 1963. Willie also investigated bo- ric acid H3BO3. Again he found an identifiable BO3 tri- angle to exist, this time with a hydrogen bonded to each oxygen. The structure is thus better written as B(OH)3. He then investigated a series of metaborates, i.e., crystals in which chemically the borate group is BO2. The metaborates he studied included in order Ca(BO2)2, KH2(H3O)2(BO2)5, K(BO2, pH(BO2), PH(BO2), Na(BO2), and Li(BO2). Again the breadth and depth of Willie's interest in each problem are obvious. Willie had suggested quite early, i.e., after his study of Ca(BO2)2, that BO2 groups do not exist in crystals. It surprised many to learn that no identifiable (BOg)"1 groups were found. The structures Willie did find in these crystals were varied and beautiful, and they clearly intrigued Willie. In Ca(BO2)2 he found the structure to consist of an end- less chain of almost perfect BO3 triangles with two oxygen atoms in each triangle shared with an adjacent triangle. He later showed how these chains were bound together by calcium atoms. He showed that the same structure existed in Li(BO2). In K(BO2), which Willie studied in 1931 and later in Na(BO2), he found the borate to again exist as a BO3 triangle, but this time three triangles were formed into a six-membered ring of B3O3 with a third oxygen bound to each boron to complete the three BO3 triangles making up the ring. In 1940 H. Tazaki found metaboric acid P- H(BO2), orthorhombic, to have the same borate structure Willie had found for K(BO2). Willie later studied the other two forms of metaboric acid. He found |5H(BO2), monoclinic, to consist of chains of borate groups, two-thirds BO3 triangles, and one-third BO4 tetrahedra. In |3H(BO2) cubic, he found all borons to be in tetrahedral configuration. Willie then went on to still more complicated metaborates. In KH2(H3O)2(BO2)5 FREDERIK WILLIAM HOLDER ZACHARIASEN 533 he found the borate structure to be a chain made up of pentaborate groups consisting of one BO4 tetrahedron bound to four BO3 groups, the groups sharing oxygens. He found 2 K2B4O7(H2O) to consist of [B4O5(OH4)]" radicals made up of two BO4 tetrahedra and two BO3 triangles having only corners in common. In Willie's last paper on metaborates, LiBO2 (1964,2), he gave preliminary results on LiBO28H2O and promised to give details later. Given more time he certainly would have done so. This example of one of Willie's areas of interest well illustrates the depth and breadth of each of Willie's investigators. Atomic and Ionic Radii In 1932 Willie published a paper entitled "A Set of Em- pirical Crystal Radii for Ions with Inert Gas Configuration," in which he compared empirically calculated values for these parameters with x-ray measurements. This paper was a needed improvement of work done by Goldschmidt and by Pauling. In the paper Willie takes into account coordina- tion number (number of nearest neighbors in the lattice), valence, and radius ratio. Starting with this paper, Willie returned again and again in later papers to refine his self- consistent table of atomic ionic radii. Such data are im- portant in correlating the behavior of differential chemi- cal elements and in making predictions of the properties of substances whose crystal structure has not yet been de- termined. Willie's last paper specifically on the subject was published in 1973, "Metallic Radii and Electron Con- figuration of the 5f-6d Metals." Glass In 1932, i.e., during Willie's first years at the University of Chicago, he departed from his study of crystals to give the first correct description of the structure of glass. He 534 BIOGRAPHICAL MEMOIRS never considered this work as very important and, on at least one occasion, had to be reminded of the work before he recalled that at one time he had written about the sub- ject. Those working on glass, however, consider his 1932 paper, which he expanded somewhat in a German version in 1933, to be the starting point of the real understanding of glass. Charles H. Green in a 1961 article on glass in Scientific American said, "The present day understanding of glass rest heavily on a single lucid paper, only twelve pages long, written in 1932 by William H. Zachariasen." Alfred R. Cooper in his introductory paper to the 1980 Borate Glass Conference said, "We dedicate this session on glass structure to Frederik William Holder Zachariasen because his single contribution to glass literature, 'The Atomic Ar- rangement of Glass,' may be the most influential paper on glass structure in this century." Before Willie's paper on glass it was said that glass con- sists of crystalline materials. More precisely, the main fea- tures of the x-ray patterns of glass could be explained on the basis that vitreous silica consists of cristobalite crystal- lites, having dimensions of about 15A and a lattice con- stant about 6.6 percent greater than in crystalline silica. Willie argued that this description could not explain the properties of glass. He then proposed a very different structure using oxide glasses as an example. Specifically, he suggested that glasses are made up of oxide groups AOn satisfying the following four rules: (1) An oxygen atom is linked to not more than two atoms A; (2) The number of an oxygen atoms surrounding atoms A must be small (re- fined in the next paragraph to triangular or tetrahedral configuration for known glasses); (3) The oxygen polyhe- dra share corners with each other, not faces (this leads to random orientation of adjacent groups and hence no long-range order, i.e., no crystalline structure); (4) At least FREDERIK WILLIAM HOLDER ZACHARIASEN 535 three corners in each oxygen polyhedron must be shared. Based on these rules, Willie was able to predict compounds which would produce a glass as well as explain many of the properties of glass. Willie's paper on glass has appeared to many who have written about it to have come out of the blue. However, Willie had discussed crystals of the rutile type XO2 in his 1926 paper with Goldschmidt, and some silicates in his 1930 paper with Bragg. From this work he had some con- cept of how SiO4 groups associate. As I have detailed in the previous section, he had also studied how borate groups fit together in a variety of ways. Thus Willie's paper on the structure of glass was simply an insightful extension of his earlier studies of groups. Liquid Structure Willie published one paper on "The Liquid 'Structure' of Methyl Alcohol" one year after his classic paper on the vitreous state (glass). Methyl alcohol was particularly in- teresting to Willie since alcohols have a tendency to form a glass. The results were consistent both with Willie's papers on the structure of glass and with Bertrum Warren's x-ray studies of the liquid state. Warren had been a postdoctoral with Sir Lawrence Bragg at the same time as Willie. The 5f Elements Perhaps Willie's most celebrated work was the elucida- tion of the nature and the chemistry of the transuranic elements. His work in this area, started in 1943 when he joined the Manhattan Project, was extremely important to the Manhattan Project. He continued to do some work in this area throughout the rest of his life. Robert Penneman of the Los Alamos Scientific Laboratory has said of this work, "No other crystalographer has done so much to ex- 536 BIOGRAPHICAL MEMOIRS pand our knowledge of heavy element chemistry, or had such a central role in the early development of atomic energy." The initial problem faced in the understanding of the chemistry and structure of the transuranics becomes clear when one recalls that during the early stages of the Manhattan Project, only microgram quantities of these ar- tificially produced elements were available. This meant that the chemistry of these elements, including the important separation processes for plutonium from its host materials, had to be determined using only these amounts. It is in- deed difficult to determine the chemical composition of a sample using ordinary chemical techniques when only mi- crogram amounts of that chemical are present. The pro- cedure adopted involved the chemists doing microchemistry on these samples, and then sending them in capillaries to Willie to find out what they had produced. As it turned out during the early stages of the Manhattan Project, in many cases the compounds produced were a complete surprise. As these studies continued, and more detailed information was obtained, the concept of these elements being a 5f series of elements, analogous to the 4f series of elements in the rare earths, grew. The experimental evi- dence for this concept rested in large part on Willie's work. He found from his x-ray studies that the radii of successive transthorium elements in isostructural compounds de- creased slowly, i.e., by about 0.03A per successive element, much as the radii of the rare earth elements decrease slowly, by about 0.03A per successive element. This was the first strong evidence for the 5f character of these ele- ments. Willie's studies were crucial in the development of the metallurgy of the transuranium elements, particularly in the important case of plutonium. Within a few months of the preparation of the first milligrams of metal, Willie recognized that the metal had several phases stable at FREDERIK WILLIAM HOLDER ZACHARIASEN 537 near-normal temperature and pressure (now a total of six). He solved the extremely complicated structure of oc-plutonium with only slide rule and insight. The x-ray pattern is complicated since the first seventeen x-ray re- flections are absent. Others had failed to solve this struc- ture using the largest computers then available. In 1952 Willie put forward one suggestion concerning these elements which has proved to be incorrect. He sug- gested that the 5f elements should be called thorides rather than actinides. The argument is in essence, are the elements predominantly trivalent, or are they tetrava- lent? In a 1961 paper that discussed the question, he said, "It is frankly admitted that the conclusions presented in this paper are somewhat speculative, and that the results as to electronic configuration ought to be based on physical properties which depend more directly on electronic interactions." His original reasoning in making this sug- gestion was based on consideration of the atomic radii ex- hibited by these metals. He had determined these radii, and they were just the ones one would expect for tetra- valent metals with five or six electrons if the 5f electrons do not contribute to bonding, an assumption generally held at the time. Later theoretical work using large com- puters showed that this assumption was not generally valid. That work and later experimental results on super- conductivity of these elements appear to have convinced Willie shortly before his death that this one suggestion was incorrect. One of the most beautiful verifications of the similarity in electronic structure of the 4f and 5f elements was the discovery in 1974 by Willie and Penneman that above 56 kbars, cerium has the same crystal structure as a-uranium. Before their insightful discovery, a-uranium had a struc- ture that was unique. 538 BIOGRAPHICAL MEMOIRS
Diffraction Theory Willie first felt the lack of adequate theory when in 1940 he wrote a paper on "A Theoretical Study of the Diffuse Scattering of X-rays by Crystals." He followed this paper with five other papers on the subject, including careful ex- perimental checks. He assigned his student Stanley Siegel the job of completing these checks. Siegal's work was pub- lished in 1941. Partly as a result of this work, Willie took on the task of going over the then extant theory. This effort resulted in his classic 1945 monograph, Theory of X- my Diffraction in Crystals. The succinctness of this book, as with many of Willie's publications, is illustrated by the story related by Wallace C. Koehler, Willie's student at the time. According to Koehler, after spending considerable time try- ing to get from one equation to the next, separated by the phrase "from this it follows," he asked Willie how he did it. Willie replied, "Don't worry, it took me two days to make that step." As this story illustrates, the book is used mainly by ex- perts in the area. Many more recent papers are little more than direct expansions of paragraphs from Willie's book. One problem that for many years made precise crystal structure determinations difficult was the lack of a method for determining the phase of structure factors. In 1952 Willie developed a method for determining the phases directly from the measured intensities. He immediately tested his method experimentally by applying it to metaboric acid. From that time his technique was refined until by 1975 over half of the structures being determined by x-ray techniques were solved by the "direct method" traceable to Willie's work. Another problem that long plagued precise structure determination was a discrepancy between the calculated and measured intensities of diffracted x-ray beams. In 1963 Willie started looking at this problem in more detail. As FREDERIK WILLIAM HOLDER ZACHARIASEN 539 one might guess from his history, his careful look at the problem began with a mineral, Hambergite, in which he found his carefully measured intensities to be incompat- ible with theory. In a reconsideration of the theory, he showed that C. G. Darwin's formula for the secondary ex- tinction correction, which had been universally accepted and extensively used, contained an appreciable error when applied to x-rays. The error was in the treatment of the polarization of the x-ray beam. In 1963 Willie published a first-order approximation for the extinction correction for a mosaic crystal of arbitrary shape. In 1965 he published two more theoretical papers in which he derived more precise formulae for extinction and multiple diffraction. Shortly thereafter he published an experimental test of his new theory using a small quartz sphere. These papers also took into account corrections necessary in highly absorb- ing crystals for the Borrmann effect. Extinction becomes more serious as the scattered intensity increases. Scatter- ing close to the incident beam is generally the most in- tense. It is just these low-angle beams which are important in the determination of valence electron distribution. With- out a precise method for taking extinction into account, determination of outer electron distributions are unreli- able. As Pepinsky said in 1975, these papers on extinction and the Borrmann effect are "a landmark in diffraction theory and broke the dam which had held back structure determin- ation. The pathway is now open to new attacks on the prob- lem of bonding electron distribution in simple structures, to far more accurate complex structure analyses, and eventually to bonding electron structure in these complex structures." Melting Points Willie published one paper in 1967, jointly with his friend Matthias and two of Matthias's associates, on "Melting-Point 540 BIOGRAPHICAL MEMOIRS Anomalies." As these authors said in their paper, "At present there is not even a satisfactory beginning of a macroscopic theory of melting. Speculative discussions, such as those given in this paper, are hence justified because they may sug- gest directions for fruitful theoretical exploration." Their suggestions involved the "partial f character of the hybrid- ized wave function of the valence band," in essence the effective number of free electrons. Willie and his friend Matthias continued to think about this problem until Willie's death, but published nothing more. Superconductivity With his friend Bernd Matthias and some of Matthias's associates, Willie was coauthor of seven papers on super- conductivity. During their years together at Chicago (1946- 48), while Matthias was an assistant professor and Willie chairman, they had many fruitful discussions, some of which contributed to Matthias's later correlations of the super- conductivity critical temperature to the effective number of valence electrons per atom. In later collaborations, among other things, Willie "interpreted correctly the nonstoichio- metric phases present in superconducting multicomponent mixtures." As these authors conclude, "The position of the elements in the periodic table, the valence electron concentration and the crystallographic structure exhibit a strong influence on the superconducting behavior." Willie's influence is clear.
SUMMARY Willie's contributions to science, and to the scientific community, have been rich and varied. FREDERIK WILLIAM HOLDER ZACHARIASEN 541 IN PREPARING THIS MEMOIR I have drawn freely from memori- als and accolades written by S. Chandraesekhar, A. R. Coo- per, J. R. Goldsmith, M. Marezio, B. T. Matthias, P. B. Moore, Linus Pauling, R. A. Penneman, Raymond Pepinsky, D. H. Templeton, and Anthony Turkevitch, and from a recorded conversation between Willie and Edward Wolo- wiec in which Willie recalls some of his history while in Chicago. 542 BIOGRAPHICAL MEMOIRS SELECTED BIBLIOGRAPHY
1925 Ueber die kristallstruktur von BeO. Nor. Geol. Tidsskr. 8:189-200. With F. Ulrich. Ueber die kristallstruktur des a- and P-CdS, sowie des wurtzits. Z. Kristallogr. 62:260-73. Die kristallstruktur der telluride von zink, cadmium und quecksilber. Nor. Geol. Tidsskr. 8:302-6.
1926 Die kristallstruktur von berylliumoxyd und berylliumsulfid. Z. Phys. Chem. 119:201-13. Notiz ueber die kristallstruktur von phenakit, willemit und verwandten verbindungen. Nor. Geol. Tidsskr. 9:65-73. Die kristallstruktur der A-modifikation von den sesquioxygen der seltenen erdmetalle. (La2O3, Ce2O3, Pr2O3, Nd2O3). Z. Phys. Chem. 123:134-50. Ueber die kristallstruktur der telluride von beryllium, zink, cad- mium und quecksilber. Mit prazisionsbestimmungen der gitter- konstanten. Z. Phys. Chem. 124:277-84. Beitrag zur frage nach dem ionisationszustand der atome im raumgitter des berylliumoxyds. Z. Phys. Chem. 40:637-41. Ueber die kristallstruktur der selenide von beryllium^ zink, cad- mium und quecksilber. Mit prazisionsbestimmungen der gitter- konstanten. Z. Phys. Chem. 124:436-48. With V. M. Goldschmidt, T. Barth, D. Holmsen, and G. Lund. Geochemi- cal distribution law on the elements. VI. Crystal structure of the rutile type with remarks of the geochemistry of the bivalent and quadrivalent elements. Nor. Vidensk. Akad.-Oslo Arbok. 1:1-21. With V. M. Goldschmidt, T. Barth, and G. Lund. Geochemical dis- tribution law of the elements. VII. Summary of the chemistry of crystals. Nor. Vidensk. Akad.-Oslo Arbok. 2:1-117'.
1927 The crystal structure of the modification C of the sesquioxides of the rare earth metals, and of indium and thalium. Nor. Geol. Tidsskr. 9:310-16. The crystal structure of MoSi2 and WSi2. Nor. Geol. Tidsskr. 9:337- 42. FREDERIK WILLIAM HOLDER ZACHARIASEN 543 Die kristallstruktur des ammoniumfluorids. Z. Phys. Chem. 127:218-24. Ueber die kristallstruktur von MoSi2 und WSi2. Z. Phys. Chem. 128:39-48. Ueber die kristallstruktur des palladiumoxyds (PdO). Z. Phys. Chem. 128:412-16. Ueber die kristallstruktur des magnesiumtellurids. Z. Phys. Chem. 128:417-20. Ueber die kristallstruktur des quecksilberoxyds. Z. Phys. Chem. 128:421- 29. 1928 The crystal structure of tetramethylammonium iodide. Nor. Geol. Tidsskr. 10:14-22. Ueber die kristallstruktur des wasserloslichen modifikation des germaniumdioxyds. Z. Kristallogr. 67:226-34. Untersuchungen ueber die kristallstrukturen von sesquioxygen and verbindungen ABO3. Nor. Vidensk. Akad.-Oslo Arbok. 4:1-165. (Ph.D. diss.) Ueber die kristallstruktur von bixbyit, sowie vom kuntlichen Mn2O3. Z. Kristallogr. 67:455-64.
1929 Bemerkungen zu der arbeit von L. Pauling: The crystal structure of the A-modification of the rare earth sesquioxides. Z. Kristallogr. 70:187-89. Notiz iieber die kristallstruktur von titanit. Nor. Geol. Tidsskr. 10:209- 12. The crystal structure of potassium chlorate. Z. Kristallogr. 71:501-16. The crystal structure of sodium chloride. Z. Kristallogr. 71:517-29. Die feinbauliche relation zwischen eudidymit und epididymit. Nor. Geol. Tidsskr. 10:449-53.
1930 With W. L. Bragg. The crystalline structure of phenacite, Be2SiO4 and willemite, Zn2SiO4. Z. Kristallogr. 72:518-28. The structure of thortveitite, Sc2Si2O7. Z. Kristallogr. 73:1-6. The crystal structure of titanite. Z. Kristallogr. 73:7-16. The crystal structure of sodium perchlorate, NaClO4. Z. Kristallogr. 73:141-46. The chemical formula of the 'zircon pyroxenes' and the 'zircon pectolite.' Nor. Geol. Tidsskr. 11:1-3. 544 BIOGRAPHICAL MEMOIRS Bemerkung zu der arbeit B. Goszner und F. Muszgnug: Ueber die strukturelle und moleculare einheit von eudialyt. Centr. Bl. Min- eral 7:315-17. The crystal structure of benitoite, BaTiSi3O9. Z. Kristallogr. 74:139-46. On meliphanite and leucophanite. Z. Kristallogr. 74:226-29.
1931 The crystalline structure of hambergite, Be2BO3(OH). Z. Kristallogr. 76:289-302. Note on the structure of groups in crystals. Phys. Rev. 37:775-76. The structure of groups XO3 in crystals./. Am. Chem. Soc. 53:2123-30. With H. E. Buckley. The crystal lattice of anhydrous sodium sulphite, Na2SO3. Phys Rev. 37:1295-1305. With F. A. Barta. Crystal structure of lithium iodate. Phys. Rev. 37:1626- 30. With G. E. Ziegler. The crystal structure of potassium chromate, K^CrC^. Z. Kristallogr. 80:164-73. A set of empirical crystal radii for ions with inert gas configuration. Z. Kristallogr. 80:137-53. Meliphanite, leucophanite and their relation to melitite. Nor. Geol. Tidsskr. 12:577-82. On the interpretation of the selective photoelectric effect from two component cathodes. Phys. Rev. 38:2290. The crystal lattice of calcium metaborate, CaB2O4. Proc. Natl. Acad. Sci. USA 17:617-19.
1932 The crystal lattice of potassium pyrosulphite, K^SgOg, and the struc- ture of the pyrosulphite group. Phys. Rev. 40:113-14. The atomic arrangement in glass./. Am. Chem. Soc. 54:3841-51. With G. E. Ziegler. The crystal structure of anhydrous sodium sulphate, Na2SO4. Z. Kristallogr. 81:92-101. Note on a relation between the atomic arrangement in certain com- pounds, groups and molecules and the number of valence elec- trons. Phys. Rev. 40:914-16. The crystal lattice of germano sulphide, GeS. Phy. Rev. 40:917-22. The crystal lattice of potassium pyrosulphite, K^Og, and the struc- ture of the pyrosulphite group. Phys. Rev. 40:923-35. Note on the crystal structure of silver sulphate, Ag2SO4. Z. Kristallogr. 82:161-62. FREDERIK WILLIAM HOLDER ZACHARIASEN 545
With G. E. Ziegler. The crystal structure of calcium metaborate, CaB2O4. Z. Kristallogr. 83:354-61. Die struktur der glasser. Gortschr. Mineral. Kristallogr. Petrog. 17:451- 52. 1933 The crystal lattice of sodium bicarbonate, NaHCO3. /. Chem. Phys. 1:634-39. Calculation of the refractive indices of sodium bicarbonate from the atomic arrangement. /. Chem. Phys. 1:640-42. Die strukture der glaser. Glastech. Ber. 11:120-23. X-ray examination of colusite, (Cu, Fe, Mo, Sn)4 (S, As, Te)34 Am. Mineral. 18:534-37.
1934 2 Note on the structure of the trithionate group, (S3O6)~ . /. Chem. Phys. 2:109-11. With R. C. L. Mooney. The structure of hypophosphite group as determined from the crystal lattice of ammonium hypophosphite. /. Chem. Phys. 2:34-37. With R. C. L. Mooney. The atomic arrangement in ammonium and cesium persulpate, (NH4)2S2Og and Cs2S2Og, and the structure of the persulphate group. Z. Kristallogr. 88:63-81. The crystal lattice of boric acid, BO3H3. Z. Kristallogr. 88:150-61. The crystal lattice of oxalic acid dihydrate, H2C2O4 . 2H2O and the structure of the oxalate radical. Z. Kristallogr. 89:442-47. The atomic arrangement in potassium trithionate crystals KgSgOg 2 and the structure of the trithionate radical (S3Og)~ . Z. Kristallogr. 89:529-37. 1935 The liquid "structure" of methyl alcohol. /. Chem. Phys. 3:158-61. The vitreous state./. Chem. Phys. 3:162-63. Note on the scattering of x-rays from fluids containing polyatomic molecules. Phys. Rev. 47:277-78. Note on the crystal lattice of samarium sulphate octohydrate. /. Chem. Phys. 3:197-98.
1936 The crystal structure of germanium disulphide./. Chem. Phys. 4:618-19. 546 BIOGRAPHICAL MEMOIRS 1937 The crystal structure of potassium acid dihydronium pentaborate KH2(H3O)2B5O10 (potassium pentaborate tetrahydrate). Z Kristallogr. 98:266-74. The crystal structure of potassium metaborate, K^BgOg)./. Chem. Phys. 5:919-22. 1938 Comments on the article by A. P. R. Wadlund: Radial lines in Laue spot photographs. Phys. Rev. 53:844.
1940 A theoretical study of the diffuse scattering of x-rays by crystals. Phys. Rev. 57:597-602. The crystal structure of sodium formate, NaHCO2. /. Am. Chem. Soc. 62:1011-13. With S. Siegel. Preliminary experimental study of new diffraction maxima in x-ray photographs. Phys. Rev. 57:795-97. Diffraction maxima in x-ray photographs. Nature 145:1019.
1941 On the diffuse x-ray diffraction maxima observed by C. V. Raman and P. Nilakantan. Phys. Rev. 59:207-8. On the theory of the temperature diffuse scattering. Phys. Rev. 59:766. Temperature diffuse scattering of a simple cubic lattice. Phys. Rev. 59:860-66. The temperature diffuse scattering maxima for rocksalt. Phys. Rev. 59:909. On the theory of temperature diffuse scattering. Phys. Rev. 60:691.
1945 Theory of X-ray Diffraction in Crystals. New York: John Wiley and Sons. 252 pp. 1947 Direct determination of stacking disorder in layer structures. Phys. Rev. 71:715-17. 1948 The UC13 type of crystal structure./. Chem. Phys. 16:254. FREDERIK WILLIAM HOLDER ZACHARIASEN 547
The crystal structure of U2Fg./. Chem. Phys. 16:425. Crystal radii of the heavy elements. Phys. Rev. 73:1104-5. Double fluorides of potassium or sodium with uranium, thorium, or lanthanum. /. Am. Chem. Soc. 70:2147-51. The crystal structure of the normal orthophosphates of barium and strontium. Ada Crystallogr. 1:263-65. Crystal chemical studies of the 5f-series of elements. I. New struc- ture types. Ada Crystallogr. 1:265-68. Crystal chemical studies of the 5f-series of elements. II. The crystal structure of Cs2PuCl6. Ada Crystallogr. 1:268-69. Crystal chemical studies of the 5f-series of elements. III. A study of the disorder in the crystal structure of anhydrous uranyl fluo- ride. Ada Crystallogr. 1:277-81. Crystal chemical studies of the 5f-series of elements. IV. The crystal structure of Ca(UO2)O2 and Sr(UO2)O2. Ada Crystallogr. 1:281- 85. Crystal chemical studies of the 5f-series of elements. V. The crystal structure of uranium hexachloride. Ada Crystallogr. 1:285-87.
1949 Crystal chemical studies of the 5f-series of elements. VI. The Ce2S3- Ce3S4 type of structure. Ada Crystallogr. 2:57-60. Crystal chemical studies of the 5f-series of elements. VII. The crys- tal structure of Ce2O2S, La2O2S and Pu2O2S. Ada Crystallogr. 2:60- 62. Crystal chemical studies of the 5f-series of elements. VIII. Crystal structure studies of uranium silicides and of CeSi2, NpSi2 and PuSi2. Ada Crystallogr. 2:94-99. Crystal chemical studies of the 5f-series of elements. IX. The crystal structure of Th7S12. Ada Crystallogr. 2:228-91. Crystal chemical studies of the 5f-series of elements. X. Sulfides and oxy-sulphides. Ada Crystallogr. 2:291-96. Crystal chemical studies of the 5f-series of elements. XI. The crystal structure of cc-UF5 and of p*-UF5. Ada Crystallogr. 2:296-98. Crystal chemical studies of the 5f-series of elements. XII. New com- pounds representing known structure types. Ada Crystallogr. 2: 388-90. Crystal chemical studies of the 5f-series of elements. XIII. The crys- tal structure of U2Fg and NaTh2Fg. Ada Crystallogr. 2:390-93. 548 BIOGRAPHICAL MEMOIRS Crystal chemical studies of the 5f-series of elements. Rec. Chem. Prog. (Spring) :47-51. With R. C. L. Mooney. Crystal structure studies of oxides of pluto- nium. In The Transuranium Elements. National Nuclear Energy Se- ries, vol. 14B, pp. 1442-7. New York: McGraw-Hill. The crystal structure of plutonium nitride and plutonium carbide. In The Transuranium Elements. National Nuclear Energy Series, vol. 14B, pp. 1448-51. New York: McGraw-Hill. The crystal structure of PuSi2. In The Transuranium Elements. Na- tional Nuclear Energy Series, vol. 14B, pp. 1451-53. New York: McGraw-Hill. Crystal structure studies of sulfides of plutonium and neptunium. In The Transuranium Elements. National Nuclear Energy Series, vol. 14B, pp. 1454-61. New York: McGraw-Hill. X-ray diffraction studies of the fluorides of plutonium and nep- tunium: Chemical identity and crystal structure. In The Transura- nium Elements. National Nuclear Energy Series, vol. 14B, pp. 1462- 72. New York: McGraw-Hill. Crystal structure studies of chloride, bromides, and iodides of plu- tonium and neptunium. In The Transuranium Elements. National Nuclear Energy Series, vol. 14B, pp. 1473-85. New York: McGraw- Hill. The crystal structure studies of sodium plutonyl and sodium neptunyl acetates. In The Transuranium Elements. National Nuclear Energy Series, vol. 14B, pp. 1486-88. New York: McGraw-Hill. The crystal structure NpO2 and NpO. In The Transuranium Elements. National Nuclear Energy Series, vol. 14B, pp. 1489-91. New York: McGraw-Hill. 1950 With S. Fried and F. Hagemann. The preparation and identifica- tion of some pure actinium compounds./. Am. Chem. Soc. 72:771- 75. With R. Elson, S. Fried, and P. Sellers. The tetravalent and pentava- lent states of protactinium. /. Am. Chem. Soc. 72:5791.
1951 Crystal chemical studies of the 5f-series of elements. XTV. Oxyfluorides, XOF. Acta Crystallogr. 4:231-36. FREDERIK WILLIAM HOLDER ZACHARIASEN 549
1952 Crystal chemical studies of the 5f-series of elements. XV. The crys- tal structure of plutonium sesquicarbide. Acta Crystallogr. 5: 17-19. Crystal chemical studies of the 5f-series of elements. XVI. Identifi- cation and crystal structure of protactinium metal and of protac- tinium monoxide. Acta Crystallogr. 5:19-21. Crystal chemical studies of the 5f-series of elements. XVII. The crystal structure of neptunium metal. Acta Crystallogr. 5: 660-64. Crystal chemical studies of the 5f-series of elements. XVIII. Crystal structure studies of neptunium metal at elevated temperatures. Acta Crystallogr. 5:664-67. A new analytical method for solving complex crystal structures. Acta Crystallogr. 5:68-73. On the anomalous transparency of thick crystals to X-rays. Proc. Natl. Acad. Sci. USA 38:378-82. Experimental crystallography. Annu. Rev. Phys. Chem. 3:359-74.
1953 Crystal chemical studies of the 5f-series of elements. XIX. The crys- tal structure of the higher thorium hydride, Th4H15. Ada Crystallogr. 6:393-95. With F. H. Ellinger. The crystal structure of samarium metal and of samarium monoxide./. Am. Chem. Soc. 75:5650-52. 1954 Crystal chemistry of the 5f-series elements. In The Actinide Elements, ed. Seaborg and Katz, National Nuclear Energy Series, vol. 14A, pp. 769-96. New York: McGraw-Hill. With R. N. R. Mulford and F. H. Ellinger. A new form of uranium hydride. /. Am. Chem. Soc. 76:297-98. With F. H. Ellinger. The crystal structure of KPuO2CC*3, NH4PuO2CO3, and RbAmO2CO3. / Phys. Chem. 58:405-8. The precise structure of orthoboric acid. Acta Crystallogr. 7:305—10. With L. B. Asprey and F. H. Ellinger. Preparation, identification and crystal structure of a pentavalent americium compound, KAmO2F2. J. Am. Chem. Soc. 76:5235-37. With P. A. Sellers, S. Fried, and R. E. Elson. The preparation of 550 BIOGRAPHICAL MEMOIRS some protactinium compounds and the metal. /. Am. Chem. Soc. 76:5935-8. Crystal chemical studies of the 5f-series of elements. XX. The crys- tal structure of tri-potassium uranyl fluoride. Ada Crystallogr. 7: 783-87. Crystal chemical studies of the 5f-series of elements. XXI. The crys- tal structure of magnesium orthouranate. Ada Crystallogr. 7: 788-91. Crystal chemical studies of the 5f-series of elements. XXII. The crystal structure of KgUF^ Ada Crystallogr. 7:792-94. Crystal chemical studies of the 5f-series of elements. XXIII. On the crystal chemistry of uranyl compounds and of related compounds of transuranic elements. Ada Crystallogr. 7:795-99.
1955 With L. B. Asprey, and F. H. Ellinger, and S. Fried. Evidence for quadrivalent curium: X-ray data on curium oxides. /. Am. Chem. Soc. 77:1707. With F. H. Ellinger, C. E. Holley, Jr., B. B. Mclnteer, D. Pavonee, R. M. Potter, and E. Staritzsky. The preparation and some proper- ties of magnesium hydride. / Am. Chem. Soc. 77:2647. With F. H. Ellinger. Crystal chemical studies of the 5f-series of ele- ments. XXIV. The crystal structure and thermal expansion of y- plutonium. Ada Crystallogr. 8:431-33. With C. E. Holley, Jr., R. N. R. Mulford, F. H. Ellinger, and W. C. Koehler. The crystal structure of some rare earth hydrides. /. Phys. Chem. 59:1226-28. With S. Fried. The chemistry and crystal chemistry of heavy element compounds. International Conference on the Peaceful Uses of Atomic Energy, Geneva, Switzerland. 1957 With F. H. Ellinger. Crystal structure of alpha-plutonium metal. J. Chem. Phys. 27:811-12. With L. B. Asprey, and F. H. Ellinger, and S. Fried. Evidence for quad- rivalent curium. II. Curium tetrafluoride./. Am. Chem. Soc. 79:5825. 1958 With B. T. Matthias. Superconductivity of rhenium nitride. /. Phys. Chem. Solids 7:98. FREDERIK WILLIAM HOLBEE ZACHARIASEN 551 With B. T, Matthias and E. Corenzwit. Superconductivity and ferro- magnetism in isomorphous compounds. Phys. Rev. 112:89. 1959 With F. H. Ellinger. Unit cell and thermal expansion of P-pluto- nium metal. Acta Crystallogr. 12:175-76. With H. A. Plettinger. Crystal chemical studies of the 5f-series of elements. XXV. The crystal structure of sodium uranyl acetate. Acta Crystallogr. 12:526-30. On the crystal structure of protactinium metal. Acta Crystallogr. 12:698- 99. 1960 With D. B. McWhan, J. C. Wallman, B. B. Cunningham, L. B. Asprey, and F. H. Ellinger. Preparation and crystal structure of ameri- cium metal. /. Inorg. Nucl. Chem. 15:185-87. 1961 With H. A. Plettinger. The crystal structure of lithium tungstate. Acta Crystallogr. 14:229-30. With M. Marezio and H. A. Plettinger. The crystal structure of gadolinium trichloride hexahydrate. Acta Crystallogr. 14:234-36. The structure of plutonium metal. In The Metal Plutonium, eds. A. S. Coffinbery and W. N. Miner, pp. 99-107. Chicago: Univer- sity of Chicago Press. 1963 With C. E. Holley, Jr., and J. F. Stamper, Jr. Neutron diffraction study of magnesium deuteride. Acta Crystallogr. 16:352-53. With F. H. Ellinger. The crystal structure of beta plutonium metal. Acta Crystallogr. 16:369-75. With H. A. Plettinger. Refinement of the structure of potassium pentaborate tetrahydrate. Acta Crystallogr. 16:376-79. The crystal structure of cubic metaboric acid. Acta Crystallogr. 16:380- 84. The crystal structure of monoclinic metaboric acid. Acta Crystallogr. 16:385-89. With M. Marezio and H. A. Plettinger. Refinement of the calcium metaborate structure. Acta Crystallogr. 16:390-92. With M. Marezio and H. A. Plettinger. The bond lengths in the sodium metaborate structure. Acta Crystallogr. 16:594-95. 552 BIOGRAPHICAL MEMOIRS
With F. H. Ellinger. The crystal structure of alpha plutonium metal. Acta Crystallogr. 16:777-83. Interpretation of monoclinic powder X-ray diffraction patterns. Acta Crystallogr. 16:784-88. With M. Marezio and H. A. Plettinger. The crystal structure of potassium tetraborate tetrahydrate. Acta Crystallogr. 16:975-80. The secondary extinction correction. Acta Crystallogr. 16:1139-44. With H. A. Plettinger and M. Marezio. The structure and birefrin- gence of hambergite, Be2BO3(OH). Acta Crystallogr. 16:1144-46. The crystal structure of palladium diphosphide. Acta Crystallogr. 16:1253-55. With Ch. J. Raub, T. H. Geballe, and B. T. Matthias. Superconductivity of some new Pt-metal compounds. /. Phys. Chem. Solids 24:1093-100. 1964 Plutonium metal. In The Law of Mass-Action: A Centenary Volume, pp. 185-94. Det Norske Videnskaps-Akakemi I Oslo. Oslo: Univer- sitetsforlanget. The crystal structure of lithium metaborate. Acta CrystaUogr. 17:749-51.
1965 Experimental differentiation between primary and secondary ex- tinction with application to radiation disorder in sodium chlorate. Acta Crystallogr. 18:703-5. Multiple diffraction in imperfect crystals. Acta Crystallogr. 18:705-10. With H. A. Plettinger. Extinction in quartz. Acta Crystallogr. 18:710-14. Dispersion in quartz. Acta Crystallogr. 18:714-16. With F. H. Ellinger. The crystal structures of PuGa4 and PuGa6. Acta Crystallogr. 19:281-83. Extinction. Trans. Am. Crystallogr. Assoc. 1:33-41.
1966 The crystal structure of Rh2Te3. Acta Crystallogr. 20:334-36. With T. H. Geballe, B. T. Matthias, K. Andres, E. S. Fisher, and T. F. Smith. Superconductivity of alpha-uranium and the role of 5f electrons. Science 152:755-57. With Robert Benz. Th3N4 crystal structure and comparison with that of Th2N2O. Acta Crystallogr. 21:838-40. FREDERIK WILLIAM HOLDER ZACHARIASEN 553 1967 General theory of X-ray diffraction in real crystals. Phys. Rev. Let. 18:195-96. With B. T. Matthias, G. W. Webb, and J. J. Engelhardt. Melting- point anomalies. Phys. Rev. Lett. 18:781-84. Theory of X-ray diffraction in crystals with stacking faults. Ada Crystallogr. 23:44-49. A general theory of X-ray diffraction in crystals. Ada Crystallogr. 23:558-64. 1968 Experimental tests of the genera} formula for the integrated inten- sity of a real crystal. Acta Crystallogr. A24:212-16. Extinction in a lithium fluoride' sphere. Acta Crystallogr. A24:324. Extinction and Borrmann effect in mosaic crystals. Acta Crystallogr. A24:421-24. Extinction and Borrmann effect in a calcium fluoride sphere. Acta Crystallogr. A24:425-27. With G. Arrhenius, E. Corenzwit, R. Fitzgerald, G. W. Hull, Jr., H. L. Luo, and B. T. Matthias. Superconductivity of Nb3(Al, Ge) above 20.5°K. Proc. Natl. Acad. Sci. USA 61:621-28. 1969 Theoretical corrections for extinction. Acta Crystallogr. A25:102. Intensities and structure factors. Concluding remarks. Acta Crystallogr. A25:276. With Robert Benz. Crystal structure of the compounds U2N2X and Th2(N,O)2X with X = P, S, As, and Se. Acta Crystallogr. B25:294-96. 1970 With F. H. Ellinger. Unit cell of the Zeta phase of the plutonium- zirconium and the plutonium-hafnium systems. Los Alamos Sci- entific Laboratory of the University of California, report no. LA- 4367:1-4. With Robert Benz. Crystal structure of the compounds U2N2X and Th2N2X with X = Sb, Te and Bi. Acta Crystallogr. B26:823-27. With A. S. Cooper, E. Corenzwit, L. D. Longinotti, and B. T. Matthias. Superconductivity: The transition temperature peak below four electons per atom. Proc. Natl. Acad. Sci. USA 67:313-19. 554 BIOGRAPHICAL MEMOIRS
With Robert Benz. Crystal structure of Th2CrN3, Th2MnN3, U2CrN3 and U2MnN3. /. Nucl. Mater. 37:109-13. 1971 Precise crystal structure of phenakite, Be2SiO4. (In Russian.) Kristallogr. 16:1161. With A. L. Bowman, G. P. Arnold, and N. H. Krikorian. The crystal structure of U2IrC2. Acta Crystallogr. B27:1067. 1972 With A. C. Lawson. L6w temperature lattice transformation of HfV2. Phys. Lett. 38A:1. With R. Benz and G. P. Arnold. ThCN crystal structure. Acta Crystallogr. B28:1724. With D. C. Johnston. High temperature superconductivity in Li-Ti-O ternary system. Mater. Res. Bull. 8:777-84. Metallic radii and electron configurations of the 5f-6d metals. /. Inorg. Nucl. Chem. 35:3487-97.
1974 With F. H. Ellinger. Structure of cerium metal at high pressure. Phys. Rev. Lett. 32:773-74.
1975 On californium metal. J. Inorg. Nucl. Chem. 37:1441-42. 1976 Bond lengths and bond strengths in compounds of the 5f elements. In Proceedings of the Fifth International Transplutonium Element Sym- posium, Baden-Baden, Germany, pp. 13-17.
1977 With F. H. Ellinger. The crystal structures of cerium metal at high pressure. Acta Crystallogr. A33:155-60. With J. P. Charvillat. Lattice parameters of the ternary compounds Cm2O2Sb, Cm2O2Bi, Am2O2Bi and Pu2(O,N)2Sb. Inorg. Nucl. Chem. Lett. 13:161-63. On the crystal structure of a'-cerium. / Appl. Phys. 48:1391-94. FREDERIK WILLIAM HOLDER ZACHARIASEN 555 1978 Crystal structure of the (X "-cerium phases. Proc. Natl. Acad. Sci. USA 75:1066-67. Bond lengths in oxygen and halogen compounds of d and f ele- ments. /. Less Common Met. 62:1-7. 1980 With R. Penneman. Application of bond length-strength analysis to 5f element fluorides. /. Less Common Met. 69:369—77. In memoriam: Walter Kauzmann (1916–2009)
Introduction
Walter Kauzmann came to Princeton in 1946 to replace his former mentor, Henry Eyring, who went back to Salt Lake City and his Mormon roots. He has written about his reminiscences in physical chemistry, proteins, and related topics in Protein Science,1 and a special issue of Biophysical Chemistry has been published containing tributes of his many students and admirers.2 These sources, together with his published work and our own personal experiences, have provided the material for this tribute and review.
In his early days, WK was at once shy and modest and a wondrous communicator. The simplicity of his laboratory–office, where this communication took place, has already been described.3 His reputation for “knowing everything” was quickly perceived by students, faculty members, and visitors. There were no limits on subject matter. Sometimes discussions involved a blistering array of quantum mechanical or statistical mechanical formulas; at other times quite basic ideas for beginning students, for example, partition coefficients or elementary discussions of the entropy. The range of topics was very large and went from mathematical science, to history, geology, paleontology, and many others. A major part of learning in these early days came from observing how a real scientist worked and thought.
In the first 5 years (1946–1951), there were at least 10 students and two postdocs associated with the laboratory. Clearly, they did not fit in his small space and most spent much of their time either in the library, their rooms, or at instruments outside the laboratory, for example, the polarimeter. He was not especially solicitous with his students. You had to have something to say to get his attention. (There is a small percentage of disgruntled students.) A disagreement or a misconception on our part was especially effective as a conversation starter.
His trips to the Carlsberg laboratory in Copenhagen, where science and fun were mixed in a very natural way, made a lasting impression on him.1 After returning from his first trip, he spent less time in his office, adopted an interest in tennis, not discussed, but obvious to his laboratory mates, and signs appeared of a lady in his life, Elizabeth Flagler, who was to become his wife in 1951.
His life was a very busy one outside of his research. One of his constant lines of interest was the structure and thermodynamics of water. Also, he published a book with David Eisenberg on water and extensive papers with Kuntz and Henn (see section on Water). He was avidly interested in his courses including those for undergraduates. A result of this interest was the publication of two excellent books4,5 at the undergraduate or beginning graduate level and one at the graduate level on quantum mechanics.6
Early Background
Walter Kauzmann was born in 1916 in the town of Mt. Vernon, NY, a small city in Westchester County. He later attended public schools in nearby New Rochelle. In 1933 he entered Cornell as a chemistry major. In his autobiographical sketch,1 he discusses at some length the effect of Walter Bancroft on his undergraduate education. Bancroft (together with G.N. Lewis and A.A. Noyes) was a major pioneer in establishing physical chemistry in the United States.7 Unfortunately, he had a basic mistrust of mathematics and quantum mechanics, which had an unfortunate and negative effect on the development of physical chemistry at Cornell. This caused WK to make a shift in interest to organic chemistry, which by contrast was an excellent part of the curriculum at Cornell.1
He chose Princeton University for his graduate career and arrived there in 1937. After a few misguided months as a major in organic chemistry, the department head, H.S. Taylor, suggested that he switch to physical chemistry and work with Henry Eyring. At this time Eyring was one of the most prominent and imaginative physical chemists in the nation sharing this distinction with Linus Pauling and John Kirkwood. Eyring was always bursting with ideas and had recently published a paper with Condon and Altar on the one electron theory of optical rotation.8 Prior to this, most theories had to do with the dynamic coupling of electronic transitions. The new theory demonstrated optical activity as arising from static, but asymmetric, perturbations from the environment of the chromophore in addition to the process of intramolecular coupling. Another paper by Condon discussed the basic theoretical side of the problem.9 These papers broadened the field considerably and generated an obvious choice of study for Eyring and coworkers. WK decided on a thesis on optical activity.
He published a number of papers based on his work as a graduate student. Two were on the fundamental theory of optical rotation,10,11 two on practical organic problems involving optical rotation, one on viscosity which probably helped to prepare him for his later viscometric studies on unfolded proteins,12 and another of a more general chemical nature. The two papers on the theory of optical rotation will be dealt with in more detail in the next section.
Optical Rotation
About the time he started his graduate studies, two new, complementary theories of optical rotation were developed, which have been cornerstones of interpretation ever since. The first was the one electron theory, which deals with the perturbation of a chromophore by the static field of its asymmetric environment.8,9 In the original paper, Condon and Eyring simply postulated the asymmetric field on a single electron (hence the name). In later applications, the chromophore was represented by a group wave function with ground and excited states. The field was approximated by calculating the charge distributions of neighboring groups, even of neutral groups.
The other theory was introduced by Kirkwood.13 It depended on the dynamic rather than static interactions of groups. In its original form, it started out with a firm basis in the quantum theory of the interaction of light and molecules and was then converted into a classical model. In its modern usage, it is quantum mechanical, and optical rotation arises from the mixed excitations on neighboring groups. It also explains effects like hypochromicity and hyperchromicity, which lead to the transfer of absorption intensity from one absorption band to another.
As a result, the field of molecular optical rotation was ripe for new development by experimentalists and theorists. WK's first paper was a survey of the literature incorporating the latest theoretical views that were made possible by the advances in theory.10 A second paper established rules for the interpretation of optical rotation and its temperature derivative: the Kauzmann and Eyring rules.11 This dealt with the fact that flexible molecules with conformations governed by statistical distribution functions would, in general, decrease the magnitude of optical rotation as the temperature increased because of the decrease in the average rotational asymmetry about bonds, whereas molecules in a rigid conformation could increase or decrease. As optical rotation is a signed quantity, one must be careful to discuss the magnitude of the rotation on making statements about temperature effects. Unfolded proteins and polypeptide chains show characteristic decreases in the magnitude of rotation as temperature is increased. Indeed in many cases, it is possible to distinguish whether a chain is folded or not by merely looking at the temperature derivative of the magnitude of rotation. Some simple facts: clupein (a highly charged unfolded polypeptide), unfolded proteins, and many peptide hormones have little or no structure. Insulin folds into a compact form but its separated A and B chains are not. This type of distinction is better done nowadays with optical rotatory dispersion (ORD), circular dichroism (CD), or NMR.
On the other hand, molecules that are locked into a conformation can have temperature slopes of either sign, depending on whether increased thermal activation tends to increase or decrease the asymmetry. With proteins, one must be careful that the measurements are not made too near the thermal transition of the protein, which strongly increases the magnitude of negative rotation. WK was quick to recognize these differences, although his work was monochromatic. From the beginning of his experiments at Princeton, he used optical rotation as one of the criteria for folded and unfolded proteins. In the early days of protein work, this correlation was very useful, but is less so now because of the battery of new techniques which are available.
One part of his reminiscence is fascinating, partly because it is so little known. WK mentions his interest in measuring the sources of optical activity in molecular systems by looking at the circular dichroism of the absorption bands.1 Awareness of this possibility can be seen in the old classical literature but little was done about it because of experimental difficulties. He discusses his interest in this type of study, and even located a unique investigator who had constructed the apparatus for the time that might have made it possible, P.A. Levine of the Rockefeller Institute. Unfortunately, Levine's apparatus had been pirated and was nonfunctional when WK visited him. This possible project was never mentioned by him in any discussions, even when CD and ORD in absorbing regions of the spectrum became popular some 20 years later. He was not interested in establishing credit for ideas that were not brought to fruition.
A last paper on the interpretation of optical rotation appeared long after most of the previous work was done.14 Although he was an authority on quantum mechanics, very little on this topic was published after his early seminal work on optical rotation and his very important book on quantum chemistry.6 Although he did not publish very much, he had a very thorough knowledge of the subject. This was probably acquired from his early connection with Henry Eyring, by teaching the Princeton graduate course in QM, and by working on optical rotation theory during his graduate student days. Regardless of where he obtained the knowledge, many of his hours at the blackboard were spent explaining topics in QM to the sincere appreciation of his own students and any graduate students, visitors, or others, who came to his door. Later, there was a series of papers on hybridization by Sovers and Kauzmann.15
Other Early Interests
The glassy state Readers of Protein Science and other biochemical journals primarily think of WK as a part of their family in terms of his seminal work on the hydrophobic interaction, protein unfolding, and other major contributions. This is probably true, but leaves out his interest in a variety of physical topics early in his career and a switch toward geophysics at the end. This overly narrow description of a man with very broad interest is easily remedied. Simply “google” the name Kauzmann. This provides just two results: the “Kauzmann temperature” and the “Kauzmann paradox,” nothing on proteins. The references are to his early work on the glassy state.16 Admittedly, the prominence of these topics comes from the fact that workers on glasses always use the Kauzmann surname in referring to his work. Nevertheless, this is evidence that he has made his mark also in fields that are quite distant from his protein work.
In a paper entitled “Kauzman(n)ia,” Cherayil discusses the sudden increase in interest in Kauzmann's work on the glassy state after some 40 years of neglect.17 Those in WK's laboratory at the time of the original publication of this paper (1948) were stunned that there was a new and large topic that we would have to learn if we were to keep up with our professor. Most of us gave up the policy of keeping up at this point. The denaturation work had switched into high gear, and the laboratory was preoccupied with other activities. The glassy state work will not be discussed in further detail, because it lies outside the interests of most readers and because of the lack of experience of the authors in this field. Modern discussions of the work will be found in the Kauzmann Festschrift.2
Another topic, which will not be taken up in detail, is the creep of soft metals.18,19
Life in the Laboratory
A real advantage to the small laboratory was the presence of the research leader right in our midst. As most of us began with no idea of what constituted real research or the nature of professors, this was a great benefit. Scientific visitors were especially interesting because the discussions were audible to all who cared to listen. Is there any better way to get to know your professor and his attitude toward research? It was especially interesting when someone like George Halsey, an Eyring student from the distant University of Washington, would implode into the laboratory, vociferous and ready for a lively discussion. Talks about science almost invariably took place at the blackboard. Those blackboard discussions added reality to the classwork acquired in courses. Princeton's graduate school was especially advanced even for present times. There were no course requirements. If you knew your material, it did not matter whether you had a course in the subject.
WK's method of introducing concepts to beginning students was especially interesting. In the early days he liked humanistic descriptions of molecular events. Molecules were not only very real but also had personalities. Many would talk of the “free energy of transfer” but Kauzmann's molecules would go into a lipid phase and cry out “Whee, I like it here.”
On the other hand this close contact was not always advantageous to the professor, who had no privacy at all during working hours. WK was not a complaining type of person, and we never heard what he thought of his overcrowded laboratory.
At this time there were no grants for faculty and no outside research fellowships for students. Apparently, there were just two Navy grants in the entire department, which supported work on the measurement of dipole moments and chemical kinetics. Many students received support from the GI Bill, which paid for tuition, books, and provided a small stipend. There were a few prestigious sounding, but slim-paying fellowships, which were reserved for the lucky. The “Harvard” fellowship paid about $700 per year. For students on the GI bill the total was more than adequate. By 1951 postdoctoral fellowships became available, and there were two in WK's laboratory by 1951. Postdoctoral fellowships for studies abroad were uncommon, but started about 1953.
But beyond these considerations, it was a great period. Students, who never expected to go to college, were moving into research. The period from 1946 to the early 1950s was one of the most exciting and galvanizing periods in American science. It became truly democratized and was the beginning of its postwar expansion. The graduate life at the Princeton Graduate College, the graduate dorm, was intellectually exciting and a wonderful experience. In his single days, WK was a frequent guest at the dinners there.
Protein Denaturation
Prior to the 1940s the word protein denaturation could mean any physical or chemical change, which would render the protein or enzyme nonfunctional.20 The idea thus included a wide variety of physical, chemical, and dissociation phenomena that are not related to structure or conformation. Neurath was one of the figures who helped to clarify the subject. One of the outcomes of the early studies of protein denaturation in the Kauzmann laboratory was to limit the concept of denaturation to the very specialized reaction of proteins, which lose their ordered, native structure (disclosed in the 1930s by the extraordinary number of X-ray reflections in studies of protein crystals, outlined by Bernal21) and become random chains. By 1948 the interesting form of denaturation was established as an unfolding reaction from the detailed specific structure and conformation of a native protein, to a random form, resembling an ordinary polymer chain. Kauzmann's laboratory played no small role in the crystallization of this idea. This specialization of the word “denaturation” greatly simplified the concept and has been the standard ever since.
It was the philosophy of the laboratory that the only sure way of establishing the nature of this transformation was by careful and complete measurements that would provide a reasonable picture of events taking place during the unfolding reaction. WK's analogy was taking apart a watch and inspecting its mechanism, if you wanted to know how it worked, even though you might not know yet how to build one. Optical rotation and viscosity were chosen as the two physical methods that would allow him to follow changes in both the conformation of the native state (optical rotation) and would allow him to detect and determine the properties of the unfolded state (viscosity). Later, he discussed in general terms the use of long-range physical properties (e.g., viscosity, sedimentation, and diffusion),22 which yield information on the extent and flexibility of unfolded molecules, and short-range properties (optical rotation, molar volume, and absorption spectra), which are sensitive to local conformation.23 At this time, NMR was a long way in the future for this type of application.
He made use of several proteins, principally bovine serum albumin and ovalbumin, with some experiments on β-lactoglobulin and pepsin. These were about the only proteins feasible at the time, at least for physical chemists who were far separated from biochemical preparation laboratories. The BSA and pepsin were purchased; ovalbumin and lactoglobulin were prepared in the laboratory. These proteins are sufficiently easy to prepare that even budding physical chemists can do it. Parameters, which were varied in the studies, were temperature, urea and guanidinium chloride concentration, pH, and added ionic compounds. Sulfate ion turned out to be especially interesting. WK had been a Princeton graduate student and as such he had a very considerable background in chemical kinetics. Because of this a great deal of the work on proteins was done on ovalbumin, which unfolded irreversibly with reaction rates that depended on denaturant concentration, temperature, pH, and the concentration of various added salts or sulfhydryl agents. These effects were studied in great detail, and the results stand as a landmark in our knowledge of the effect of chemical and physical parameters on protein reaction rates. The results of these studies appeared in 1953 as a monumental series.24–28
It was not known at the time that most simple proteins can be unfolded in a reversible reaction, which permits the study of the folding and unfolding reaction and the investigation of the thermodynamics of folding (ΔH, ΔS, ΔG, and ΔV) as functions of temperature and pressure. Serum albumin is a reversible system, but in the early studies was looked upon with a certain amount of suspicion, because it clearly contained organic contaminants in its commercial form. WK liked to demonstrate this by putting the protein into concentrated urea, whereupon a strong odor of organic compounds would develop. These were apparently buried tightly in the folded protein. Pepsin was also studied, and it was shown that this protein is not unfolded by urea under ordinary conditions of pH and temperature.
Between the start of these studies to the published results in 1953, 7 years passed. They were given a great deal of thought because they were probably the most elaborate series of studies on protein denaturation that had been produced, a record that might still stand. The students involved were not damaged by the delay. These were more leisurely times and it was not of vital importance that a new Ph.D. student should publish immediately. The “old boy” system, direct contact between research professors, was still active. Despite the serious disadvantages of this procedure, it could work well for the placement of able students who had research ability.
The importance of these studies was generally known to the world of protein and physical chemistry long before their publication. I can recall visiting the Cal. Tech. Chemistry Department in 1951. As soon as it was learned that I was a Kauzmann student, I was immediately asked to give a seminar on the laboratory's latest results on protein unfolding. With the series of papers cited above, WK had established himself as a leader in the field of protein physical chemistry. His studies made it clear that having SH groups and SS groups in a protein chain added complicated interchanges in the unfolded protein chain.29 WK used this discovery to study the nature and kinetics of disulfide interchange, , where the R factors are protein chains or protein moieties. Later, there was a considerable effort to find unfolding reactions that were reversible and directly led to the thermodynamics of unfolding. Kauzmann was a pioneer in this subject also with his studies of the reversible unfolding in other proteins (see the series of papers quoted above).
The Carlsberg Laboratory
In 1949, WK informed his laboratory group that he would be taking a half-sabbatical leave for a visit to the mountains and glaciers of Switzerland, a stay in Cambridge, England to attend a meeting and visit scientists, and a working visit to the Carlsberg laboratory in Copenhagen. This trip and its various adventures are recounted in Kauzmann's later reminiscences.1
At this time, none of us had heard of the Carlsberg laboratory, but his discussion of its three leaders from Kjeldahl, to Sorensen to Linderstrøm-Lang convinced us that it was a good place for him to go. He would first arrive in the summer for a trip to Switzerland and a walking tour of its mountains and glaciers.
At this point, it is worth mentioning that although the Carlsberg laboratory was supported by the Carlsberg Brewery, it was by way of the Carlsberg Foundation, a nonprofit organization established about 1900. Its main mission was the support of scientific research, the support of a number of Danish art and cultural museums, and the education and support of young scientists and scholars on their way to their careers. It is true that there was an extensive selection of free beer in the laboratories, but that was the only direct connection.
He must have had a wonderful time at the Carlsberg laboratory. They were certainly enthusiastic about him. Lang, the laboratory Professor, was genial and amusing as well as being a great scientist. He played the violin, painted in oils, was a born raconteur, a connoisseur, a writer and had an incredible string of jokes at his disposal, not always high- minded. Lang and the laboratory made a deep impression on WK. The same can be said for the heritage his visit made on the Carlsberg laboratory. A number of years later, Martin Ottesen, the assistant director and later the director of the laboratory, was still reminding visitors of Kauzmann's statements that one should never forget the excluded volume in presenting results on solution thermodynamics.
He had two visits to the Carlsberg laboratory, one in 1949 and the other in 1957. In both instances, remarks by Linderstrøm-Lang led him to fruitful research. During the first, he mentioned the precipitates that formed when serum albumin was unfolded by urea. Lang remarked that the addition of kerosene to urea solutions led to massive precipitation. As a result, WK turned to experimental work. His conclusion after a study of every available chain compound in the laboratory was that urea would form a crystalline product with all straight- chain organic liquids, including those with heteroatoms like diethyl ether, but would not form crystals with branched chains. The addition of urea solutions thus provided a way of separating branched from unbranched chains. But, alas, Max Møller of the Carlsberg laboratory, found a 1949 paper on the subject, which reported the phenomenon in detail including a crystal structure of the adduct.30 WK was of course disappointed that things turned out this way, but not overly so. He had gotten hold of an experimental problem and solved it all by himself. In his reminiscences he goes out of his way to mention the predecessors of his work on this problem.1
Evidently, WK returned from his first visit to the Carlsberg laboratory with the intention of broadening his activities outside the laboratory and research. Within roughly a year of his return, he had taken up tennis and bought an automobile (the new and pace-setting Studebaker). Not long afterward, he became engaged and married to Elizabeth Flagler, a research associate of the Biology department, who had already made her mark in research at the university.
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The Hydrophobic Bond
The 1954 paper
In the 40s and early 50s, it was well known that the nonpolar parts of protein chains must play a part in the overall structure of proteins. They were included in every list of interactions. Also, well known was the old rule from organic chemistry that neither aliphatic chains nor aromatic molecules “liked” to be in an aqueous environment. It was apparent that bringing nonpolar chains together in the structure of a protein would lower the free energy and promote the folding reaction. The attractive energies were assumed to come from London dispersion forces, which had provided good theories of the interaction of hydrocarbons with one another in the gaseous phase and nonaqueous solution phases. This was about as far as theories went for the attractive forces of nonpolar groups in proteins: London forces between nonpolar groups contributed to the stability of proteins but presumably in a conformation-dependent way, as a leucine sidechain can interact with other nonpolar and polar groups via many pairwise conformations.
This is far different from the situation with hydrogen bonds. By this time, Pauling et al. had done their work and Linderstrøm-Lang had introduced the concept of the primary, secondary, and tertiary structure of proteins, which is now extensively used. The primary structure is essentially the sequence of amino acids, the secondary structure is a listing of the presence, location, and extent of the Pauling-Corey structures: mainly α-helices and pleated sheets,31 and later various “turns.” The tertiary structure is everything else seen in a successful determination of protein structure: the location and structure of disordered regions, the location of SS and other cross bridges, the location of all interactions amongst the amino acids sidechains including their pairwise geometry, and so forth. Solution methods were available to demonstrate the presence of secondary structures in proteins. The determination of tertiary structure requires a complete structure determination, and information has slowly accumulated over the years by means of X-ray diffraction.
In the beginning there was very little consideration of the interaction of nonpolar groups with an aqueous environment. This can be seen in the Kauzmann paper published in 1954,22 where the entropy of unfolding is derived mainly from the liberation of high entropy random chains. Apparently, at the time, WK was only thinking about the consequences of the removal of the aqueous shell from nonpolar molecules, which occurs simultaneously with folding. His first idea was that the interaction of a nonpolar group with an aqueous environment was energetically repulsive because the nonpolar group prevents the formation of the full complement of H-bonds in its solvation shell. In the 1954 paper, no entropic effects are attributed to the hydration shell, which plays a purely energetic role: the breaking of H-bonds in the shell.
As WK states in his reminiscences, this idea was tried out on Linderstrøm-Lang at a time when the latter was very ill. Lang, as usual, looked at the problem in the most direct and simple manner possible. According to Kauzmann, he said, “That's strange. When you mix alcohol with water, heat is given off!” This indicated that the energy of this system was lowered, not raised, on the contact of hydrophobic molecules and water, A distinct call back to the drawing board.
The 59 paper
WK found the answers he was seeking in the older literature, especially in the work of Frank and Evans32 and Butler.33 His earlier notion was not correct. Addition of nonpolar groups to water causes a decrease in the energy and the entropy of the contact region. Both vary with temperature. The thermodynamic force is essentially entropic as ΔH goes to zero for many hydrocarbons at temperatures near to 20°. There, it is purely entropic, ΔG = −TΔS (ΔH = 0). At other temperatures, ΔG is both entropic and enthalpic with entropy dominating in the temperature range where the hydrophobic interaction is most effective. This is a reasonable summary of the results, which can differ depending on whether the aliphatic molecule is transferred from the gas phase, the pure liquid, or a solution in a different solvent.23
WK's approach is mainly qualitative in this pioneering paper. If nonpolar surfaces in contact with water are sources of positive free energy, then the elimination of such surfaces by the folding of a protein will lead to negative free energies, which thermodynamically contribute to the folding free energy. As is well known, there are hundreds of papers that have followed this first one on the hydrophobic interactions and a number of them are discordant. We have preferred to stick to the early and simple version of the initial paper to provide a guide to WK's thought processes in their creative stage. A long review paper on the hydrophobic interaction was published a number of years ago.34 When asked what he thought of this review paper with its 371 references, Kauzmann replied, “Do you know, sometimes I think that I no longer understand the hydrophobic bond.”
The concept of hydrophobia now seems reasonably clear and intuitive but details vary; the current situation is complex and not entirely resolved.
Water
Since his reading as a graduate student in the late 1930s, Kauzmann had developed a life-long interest in water and other hydrogen-bonded liquids. Understanding the unusual thermodynamic and transport properties of liquid water became even more important to him as he studied proteins and formulated his ideas on the hydrophobic bond (later referred to as the hydrophobic interaction). He used to say that we could never expect to understand the chemistry of proteins completely without understanding aqueous solutions and the structure of water—the universal biological solvent and liquid of life.
Starting in the 1960s, WK's group undertook more specific studies directed toward understanding water and aqueous solutions, doing basic work, for instance, on NaOH solutions,35 solute–solute interactions in water,36 effect of pressure on spectral solvent shifts in water,37 the effects of temperature and pressure on the solubility of 4-octanone in water,38 and solutions of water in organic solvents.39
In the mid-60s, David Eisenberg joined WK's group as a postdoc and collaborated in the writing of the well-known monograph on the structure and properties of all the physical states of water.40 Their book, originally published in 1969, was printed in several languages and continued to be printed 30 years after its initial release.
As most readers know, there has been a longstanding debate, going back to Roentgen in 1892, about the structure of water. Does this unusual liquid consist of an equilibrium mixture of discrete, variously bonded molecules, or is it made up of molecules possessing a more or less uniform, continuous distribution of energies (and configurations), as is true of other liquids.
Notwithstanding the fact that his graduate advisor and mentor, Henry Eyring, had published on a mixture-type model of water,41 WK was firmly in the camp that water did not consist of a mixture of species. He preferred a random network with distorted hydrogen bonds as a more accurate model. His thinking on the subject probably goes back to his reading of the classic paper of Bernal and Fowler.42 It was also clear that WK was an admirer of Pople's pioneering work on a distorted, H-bonded network to account for the properties of water.43 In 1976, WK published a highly cogent, but underappreciated paper,44 that basically shows how the mixture models of liquid water cannot account for its unusual thermodynamic properties. For instance, to explain the temperature and pressure dependences of water's compressibility and thermal expansion coefficient, one would need a large cluster of x molecules, while to explain the temperature dependence of the heat capacity and Raman spectrum, a small cluster of y molecules would be required. The paper was published in a journal not widely read by physical chemists, and probably did not gain the audience it deserved.
As he neared retirement, WK phased out all projects requiring experimentation. Indeed, he relinquished all of his laboratory space in 1980 and focused the last of his academic research solely on theoretical studies and analysis of existing data. Now, when trying to understand and explain physical phenomena theoretically, probably the majority of WK's students would agree that he preferred to start with a heuristic model, and then build from there to a theoretical explanation of physical phenomena. This is how he approached liquid water.
WK had his last grad student (ARH) review almost all the models, mixture and otherwise, that had been published on water up to that time (1978), not to mention all the computer simulations that had been carried out. Their initial effort attempted to expand Pople's model and quantify some of the particularly interesting PVT properties of water, such as the volume dependence of the heat capacity. However, it became apparent that too many parameters had to be introduced to fit the data. This was a criticism of many previously proposed models, and it was something that WK wanted to avoid.
When Rice and Sceats introduced their random network, distorted ice lattice-type of model for water around the same time (see ref.45 and references therein for a review), WK recognized it as being in line with his own thinking and considered it a major contribution to the field. Rice and Sceats, however, had not attempted to explain the pressure–volume properties of water, and it fell to us to modify and expand the distorted ice lattice model to encompass these unique properties. This was accomplished by considering a small, spherical sample volume of water whose density varied according to the H-bond distortion variables of stretching and bending. With a complete expression of the model's Helmholtz free energy in terms of the H-bond distortion variables now in hand, the free energy could be minimized with respect to those variables over a range of volumes and temperatures and the thermodynamic properties derived therefrom. WK's long-held view that water is more likely a random network of distorted hydrogen-bonded molecules culminated, after his retirement, in the publication of a random network, continuum model that successfully addressed numerous thermodynamic properties of water.46,47
Other Activities
This section will examine a few of his WK's activities outside his research and teaching.
About the time of his marriage in 1951, the Kauzmanns initiated their annual trips to Cape Breton Island in Nova Scotia for the summer. He accepted no remuneration for this annual trip. He spent much time caring for the property, construction, and doing odd jobs. A large fraction of his book writing took place during these periods.
In addition to his research and teaching at Princeton, he was Chairman of the Chemistry Department and later of Biochemical Sciences. He was a strong recruiter for science at Princeton. He had a special interest in bringing the new biology and accomplished biological scientists to his University.
His honors include membership in the National Academy of Sciences and the American Academy of Arts and Sciences, receipt of the first Linderstrøm-Lang Award, an honorary degree from the University of Stockholm, the Stein and Moore Award, the annual Kauzmann Lecture at Princeton, a festschrift in his Honor with 48 papers by students and admirers,2 and key official roles in many conferences on biophysical chemistry.
One often sees his deepest thought and interpretation in his reviews: the glassy state16 (also see Cherayil17 for a much later and very positive review), his set of five papers in 1953 on protein unfolding and denaturation,24–28 protein denaturation and interactions,22 the hydrophobic interaction,23 and quantum chemistry.6 He has summed up his own history.1
References
1. Kauzmann W. Reminiscences from a life in protein physical chemistry. Protein Sci. 1993;2:671–691. [ 2. Henn AR, editor. Vol. 105. Amsterdam: Elsevier; 2003. Special issue in honour of Walter J. Kauzmann; pp. 153–755. Biophysical chemistry. 3. Schellman JA. The Kauzmann lab in the late 1940s. Biophys Chem. 2003;105:161–165. 4. Kauzmann W. Thermal properties of matter. Vol. 1. New York, NY: Benjamin; 1966. Kinetic theory of gases. 5. Kauzmann W. Vol. 2. New York: Benjamin; 1967. Thermodynamics and statistics with applications to gases. Thermal properties of matter. 6. Kauzmann W. Quantum chemistry. New York: Academic Press; 1957. 7. Servos JW. Physical chemistry from Ostwald to Pauling. Princeton, NJ: Princeton University Press; 1990. 8. Condon EU, Altar W, Eyring H. One-electron rotatory power. J Chem Phys. 1937;5:753– 775. 9. Condon EU. Theories of optical rotatory power. Rev Mod Phys. 1937;9:432–457. 10. Walter J, Eyring H, Kauzmann W. Theories of optical rotatory power. Chem Rev. 1940;25:339–407. 11. Kauzmann W, Eyring H. Effect of rotation of groups about bonds on optical rotatory power. J Chem Phys. 1941;9:41–53. 12. Schachman HK, Kauzmann WJ. Viscosity and sedimentation studies on tobacco mosaic virus. J Phys Colloid Chem. 1949;53:150–162. 13. Kirkwood JG. The theory of optical rotatory power. J Chem Phys. 1937;5:479–491. 14. Kauzmann W, Clough FB, Tobias I. Principle of pairwise interactions as a basis for an empirical theory of optical rotatory power. Tetrahedron. 1961;13:57–105. 15. Sovers O, Kauzmann W. Role of d hybridization in the pi molecular orbitals of unsaturated hydrocarbons. J Chem Phys. 1964;40:1165. 16. Kauzmann W. The nature of the glassy state and the behavior of liquids at low temperatures. Chem Rev. 1948;43:219–256. 17. Cherayil B. Kauzman(n)ia. Curr Sci. 1995;68:777. 18. Slifkin LM, Kauzmann W. The creep of zinc single crystals. J Appl Phys. 1952;23:746– 753. 19. Tanenbaum M, Kauzmann W. The creep recovery and annealing of zinc single crystals. J Appl Phys. 1954;25:451–458. 20. Neurath H, Greenstein JP, Putnam FW, Erickson JO. The chemistry of protein denaturation. Chem Rev. 1944;34:157–264. 21. Bernal JD. Structure of proteins. Nature. 1939;143:663–667. 22. Kauzmann W, McElroy W, Glass B, editors. Symposium on the mechanism of enzyme action. Baltimore: McCollum-Pratt Institute, Johns Hopkins University; 1954. Denaturation of proteins and enzymes; pp. 70–110. Contribution No. 70; discussion 110–120. 23. Kauzmann W. Some factors in the interpretation of protein denaturation. In: Anfinsen CB, Anson ML, Bailey K, Edsall JT, editors. Vol. 14. New York: Academic Press; 1959. pp. 1– 63. Advances in protein chemistry. 24. Simpson RB, Kauzmann W. Kinetics of protein denaturation. I. Behavior of the optical rotation of ovalbumin in urea solutions. J Am Chem Soc. 1953;75:5139–5152. 25. Schellman J, Simpson RB, Kauzmann W. Kinetics of protein denaturation. II. Optical rotation of ovalbumin in solutions of guanidinium salts. J Am Chem Soc. 1953;75:5152– 5154. 26. Frensdorff HK, Watson MT, Kauzmann W. Kinetics of protein denaturation. IV. Viscosity and gelation of urea solutions of ovalbumin. J Am Chem Soc. 1953;75:5157–5166. 27. Frensdorff HK, Watson MT, Kauzmann W. Kinetics of protein denaturation. V. Viscosity of urea solutions of serum albumin. J Am Chem Soc. 1953;75:5167–5172. 28. Kauzmann W, Simpson RB. Kinetics of protein denaturation. III. Optical rotations of serum albumin, beta-lactoglobulin, and pepsin in urea solutions. J Am Chem Soc. 1953;75:5154–5157. 29. Kauzmann W, Douglas RG., Jr Effect of disulfide bonding on the solubility of unfolded serum albumin in salt solutions. Arch Biochem Biophys. 1956;65:106–119. 30. Schlenk W. Die Harnstoff-Addition der Aliphatischen Verbindungen. Ann Chem Leipzig. 1949;565:204– 240. 31. Pauling L, Corey R, Branson H. The structure of proteins: two hydrogen-bonded helical configurations of the polypeptide chain. Proc Natl Acad Sci USA. 1951;37:205–211. 32. Frank H, Evans M. Free volume and entropy in condensed systems. III. Entropy in binary liquid mixtures; partial molal entropy in dilute solutions; structure and thermodynamics in aqueous electrolytes. J Chem Phys. 1945;13:507–532. 33. Butler JA. The energy and entropy of hydration of organic compounds. Trans Faraday Soc. 1937;33:229–238. 34. Blokzijl W, Engberts J. Hydrophobic effects. Opinions and facts. Angew Chem Int Ed Engl. 1993;32:1545–1579. 35. Bodanszky A, Kauzmann W. The apparent molar volume of sodium hydroxide at infinite dilution and the volume change accompanying the ionization of water. J Phys Chem. 1962;66:177–179. 36. Kozak JJ, Knight WS, Kauzmann W. Solute-solute interactions in aqueous solutions. J Chem Phys. 1968;48:675–690. 37. Zipp A, Kauzmann W. The anomalous effect of pressure on spectral solvent shift in water and perfluoro n-hexane. J Chem Phys. 1973;59:4215. 38. Kim K, Kauzmann W. Concentration dependence of the specific volumes of human oxyhemoglobin, bovine serum albumin, and ovalbumin solutions. J Phys Chem. 1980;84:163–165. 39. Henn AR, Kauzmann W. New considerations of the Barclay-Butler rule and the behavior of water dissolved in organic solvents. Biophys Chem. 2003;100:205–220. [PubMed] 40. Eisenberg DS, Kauzmann WK. The structure and properties of water. New York: Oxford University Press; 1969. 41. Jhon MS, Grosh J, Ree T, Eyring H. Significant-structure theory applied to water and heavy water. J Chem Phys. 1966;44:1465–1472. 42. Bernal JD, Fowler RH. A theory of water and ionic solution, with particular reference to hydrogen and hydroxyl ions. J Chem Phys. 1933;1:515–548. 43. Pople JA. Molecular association of liquids. II. A theory of the structure of water. Proc R Soc A. 1951;205:163–178. 44. Kauzmann W. Pressure effects on water and the validity of theories of water behavior. Colloq Int CNRS. 1976;246:63–71. 45. Rice SA, Sceats MG. A random network model of water. J Phys Chem. 1981;85:1108– 1119. 46. Henn AR, Kauzmann W. Equation of state of a random network, continuum model of liquid water. J Phys Chem. 1989;93:3770–3783. 47. Henn AR. The surface tension of water calculated from a random network model. Biophys Chem. 2003;105:533–543.
CONTEMP. PHYS., 1969, VOL. 10, NO. 5, 473-488
Under What Conditions can a Glass be Formed?
DAVID TURNBULL Division of Engineering and Applied Physics, Harvard University Cambridge, Massachusetts 02138
SUMMARY.Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, - $, the liquid volume, w], and the seed density, ps and upon a set of materials constants: the reduced crystal-liquid interfacial tension, a, the fraction, f, of acceptor sites in the crystal surface, and the reduced glass temperature, Tq. The glass-forming tendency will be greater the larger are - T,a and T,, and the smaller are vl. py, andf. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass tempera- ture. There are at least some glass formers in every category of material, according to bond type, i.e. covalent, ionic, metallic, van der Weals or hydrogen. However, it is not established whether or not every substance can be put into a glass form. 1. Introduction Glass is considered by technologists to be any solid formed by the continuous hardening of a cooled liquid. It is to be distinguished from a solid formed by discontinuous solidification in which solid masses appear and grow in the liquid, the solidification occurring only at the liquid-solid interfaces. By hardening we mean increasing resistance to forces causing the body to flow and permanently change its shape. The quantitative measure of this resistance is the shear viscosity. This property changes continuously upon the transition of a liquid to a glass. Therefore it is necessary to make some rather arbitrary choice of the viscosity value above which a body is considered to be a solid rather than a liquid. The viscosities of most common liquids, e.g. water, alcohol, mercury, at room temperature are of the order of Ns mP2= 10-2 poise.? For comparison, the approximate viscosities on
Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 the same scale of some other common substances are: air, 1 Ow4; lubricating oil, 1; pitch, lo6. The viscosity value chosen for distinguishing fluid from solid behaviour is commonly taken to be 1014N~m-2=1015poise. m7e may see what this means by applying the generalization that the average time, rr,for a rearrangement of a molecular position in the liquid seems to scale roughly as the viscosity. At a viscosity of rr is about 10-l2 s so that at the liquid- glass transition it is about lo5 s (or one day). Thus we consider a body to be solid if no permanent change in its shape is apparent upon the application of a small force for one day. It is evident that, to change from poise to 1015poise, the viscosity must increase very rapidly over some part of the temperature range in the transition from liquid to glass. In fact, the viscosities of glass-forming liquids are fairly well described, at least between and lo7poise, by the Fulcher equation,(l) which has the form: rl =A exp [al(T - To)] (1) 7 poise=10-1 Ns m-2 474 David Turnbull
where A, u and Toare constants depending on the material and TK is the absolute temperature. When To= 0 the equation assumes the familiar Arrhenius form. 7 will increase very rapidly with falling temperature either when a is very large relative to T or, if a is small, when T has fallen nearly to To. The viscosities of pure silica or of germania (GeO,) are quite well described(,) by an
Arrhenius equation (ToN 0) with an activation energy which is very large and of the order of the chemical bond energy in the structure. In contrast, the viscosities of such glass formers as toluene or isobutyl chloride are described(4 by rather small values of a but with To’swhich are a substantial fraction (e.g. 4 to $) of the thermodynamic crystallization tem- perature. T,. This means that the viscosity is low and increasing slowly with falling temperature above Tm,but it then increases with extreme rapidity when the temperature falls nearly to To. The change from the quite fluid to the solid condition, characteristic of the glass, then occurs over quite a narrow temperature interval above To. The isobaric viscosity-temperature behaviour of such a glass-forming substance in its various states is shown schematically in fig. 1. Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010
Fig. 1. Variation of the logarithm of the shear viscosity in poise with reciprocal reduced temperature when the liquid is cooled from the liquid to the glass (solid) state. The upper curve is for fused silica2 and T, is 1 at the absolute melting point of cristobalite, 1993 K. The lower curve has a form typical for simple molecular glass formers such as, i-butyl bromide or o-terphenyl.
It would seem preferable to distinguish glasses from other solids by their properties rather than by their mode of formation. In particular, the molecular structure of glasses, as determined by diffraction, always appea,rs to be amorphous while that of solids formed discontinuously is crystalline. However, the definition that glasses are simply amorphous solids has not Under What Conditions can a Glass be Formed? 475
gained wide acceptance. One reason for this is that the properties, such as density and heat content, of an amorphous solid, at specified composition, temperature and pressure, vary somewhat with the method of its preparation. It is fair to say, however, that this property variation occurs in glasses as ordinarily defined as well as in amorphous solids formed, for example, by deposition from a vapour or electrolytically from a liquid solution. A more basic difficulty with the structural definition is that the diffraction behaviour of a crystalline solid after continuous refinement of its crystallite size becomes qualitatively very similar to that exhibited by a glass. From this is might appear that glasses are simply crystalline solids with extremely small (e.g. three or four molecular diameters) grain sizes; that is, duplex mixtures of highly ordered and highly disordered material. However, Za~hariasen(~)showed long ago that when the atomic co-ordination number is sufficiently small, atoms can be assembled together in a continuous random network which can properly model a glass. This was made especially clear by the demonstration of Bell and Dean(5)that the radial distribution function, density and configurational entropy of silica glass are in remarkably good accord with a model constructed on the basis of Zachariasen’s concepts. Grigorivici’s(6)model for elemental amorphous germanium also is of the Zachariasen type. Recently Professor Bernal and his associates(’) have shown that even uniform hard spheres can be assembled into a dense continuous random structure which apparently does not collapse, when uniformly com- pressed, into a crystalline arrangement. In both the Bernal and Zachariasen type structures, local molecular configurations in which the idealized symmetry is non-crystallographic, e.g. pentagonal, are prominent. The density of an assembly of randomly oriented crystallites should decrease with crystallite size, owing to the density deficit at crystallite boundaries, and fall below the density of the continuous random structure at some critical value, dc, of the average crystallite size. d, is difficult to estimate@)but, from inspection of models, it appears to be of the order of 10 sphere diameters for hard spheres. These structural considerations have led to the concept of an ‘ideal’ glass state developed for polymeric systems by Gibbs and DiMar~io(~)and for simpler and even monoatomic systems by Cohen and the author.(lO) An ideal glass Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 is a solid in internal equilibrium in which there is, just as in a crystalline solid, a definite set of equilibrium positions about which the atoms oscillate. How- ever, in contrast with crystalline solids, the set of equilibrium positions in an ideal glass does not exhibit translational symmetry; that is, they do not fall on a periodic (or repeating) pattern. We might say that an ideal glass is a solid which exhibits an infinite unit cell. It is often supposed that the structure of glasses, as ordinarily defined, will approximate to that of the ideal glass while amorphous solids formed by deposition methods are likely to be microcrystalline. However, whether or not this distinction between the two types of amorphous solids actually exists, in general, is still an unresolved problem. For the purposes of this article any body, however formed, which exhibits diffraction behaviour consistent either with the continuous random model or with a microcrystallite model, in which the average crystallite size appears to be no greater than about five molecular diameters, will be considered to be a glass. The conditions for the formation of glasses so defined will be set forth. 476 David Turnbull
The first question to consider is, is glass ever the most stable state of a substance? In particular, is a solid more stable in a glass or in a crystalline state; or, if the crystal is the more stable at low temperature, is the temperature, Tm, at which the crystal and the amorphous state are in equilibrium, ever lower than the temperature, Tg,at which the liquid hardens to a glass? There seems to be no rigorous theoretical proof that the most stable form of a sub- stance, even if it is monatomic, must at low temperature be crystalline rather than glassy or liquid. However, experience has shown that,,with the exception of helium, the most stable low temperature forms of pure substances are crystalline. Also all the observed glass formation temperatures of pure sub- stances are lower than their equilibrium crystallization temperatures ( Tm) even though, again, there is no rigorous theoretical requirement that this be so. In particular, most liquids exhibit a viscosity of the order of 10-2 poise at their Tm and the closest approach at Tm to the liquid-glass transition viscosity, 1015 poise, seems to be by silica with lo7poise (see fig. 1). Since, according to the experience just cited, substances are always more stable in crystalline than in glassy form, the question, ‘Under what condit,ions can a glass be formed?, becomes ‘Which substances can exist in glassy form and under what conditions can the crystallization of these substances be bypassed?’ A closely related question is, ‘Even if a glass has formed, why does it persist for very long periods, e.g. at least for thousands of years in some relics ? ’ Concerning the capability of substances to exist in glass form, the ‘ideal’ glass model, already alluded to, predicts that all liquids would form glasses when sufficiently undercooled. Experience is as yet too limited to show if this prediction is generally valid. Liquid helium definitely does not form a glass at ambient pressure, but this is due to quantum effects which are expected to be much less important in the flow behaviour of other liquids. Thus, to form a glass, a material must be brought to its glass temperature, if it has one, without crystallization. This might be achieved by depositing the material from a solution on to a cold substrate held below its glass tem- perature or by cooling its melt. The conditions for cooling a melt to a glass will be considered first. In this case the probability that a glass forms is Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 greater the smaller is the crystallization rate and the larger is the cooling rate.
2. Rate of crystallization of liquids The crystallization of a fluid, liquid or gas, occurs by the formation, called ‘nucleation’, of crystallization centres (‘nuclei’) and the growth of these centres at the expenw of the adjacent fluid. Hence it is, as noted at the beginning of this article, confined entirely to the interfaces where the crystal and fluid meet. This means that, at any instant, only a minute part of the material in a body is in a position to crystallize. For example, this part is only 1 in lo7 to los if the total area of crystals growing in 1 em3 of material is 1 em2. In contrast, glass forms homogeneously by the rearrangement of molecules throughout the entire body. However, the extraction of heat which usually motivates glass formation is normally through the enternal surfaces of the body. The crystallization rate of an undercooled liquid is then specified by the rate of crystal nucleation and by the speed, u,with which the crystal-liquid interface advances. Both of these rates are strongly dependent upon the Under What Conditions can a Glass be Formed? 477
reduced temperature, Tr, and undercooling which are defined as:
where T, and T are, respectively, the equilibrium crystallization and actual absolute temperature. The resistance of a liquid to nucleation, as measured by the undercooling necessary to overcome it, is much greater than its resistance to growth. For example, long ago, Fahrenheit observed that, to initiate its freezing, water often had to be undercooled 10 to 20 K or more, but, once begun, freezing continued at a temperature only slightly less than T,. The water was warmed to this temperature immediately after nucleation by ‘recalescence’, i.e. the release of the heat of crystallization by the rapidly growing ice crystals. We expect that the velocity of the crystallization front should be inversely proportional to an average jump time, 71, of the molecules in the interfacial region and directly proportional to some function, f(ATr),of the undercooling which motivates crystallization. This function must increase with AT, from zero at ATr=O; at small ATr it is linear in the limit that all crystal surface sites are suitable for the reception of molecules and it rises as a higher power of AT, in the other cases. A plausibility argument,(ll) but not proof, can be given that 71 should scale as the shear viscosity in the crystallization of molecules. This argument is that growth is limited essentially by molecular rearrangement in the liquid adjacent to the interface which positions the molecule properly for attachment to the crystal. Indeed, experience indicates that the growth rate in liquids with 77 3 1 poise is approximately described by:
where k, is a constant which is specified in the various models. In liquids with smaller viscosities, u is so large that the continued freezing becomes limited principally by the rate of dissipation of the heat of crystallization and it becomes very difficult to specify AT, at the solid-liquid interface. Eqn. (2) predicts that u should be of the order of 1O1O to loll molecular Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 spacings per second, depending on the model and the system, at 77 = 10-2 poise and ATr = 0.1. For nickel under these approximate conditions Walked1% measured UNloll atom spacings/second (5000 cm s-l). At the liquid-glass transition viscosity, 1015 poise, and ATr=0*1 eqn. (1) predicts that u should be only 1 molecular spacing per 10 to 100 days. This result indicates why glasses, even though less stable than the crystallized states, persist for very long periods. Even if well seeded with crystal nuclei, a glass with a viscosity greater than 1020poise should not, according to eqn. (1)) crystallize appreciably in 10 000 years. In fact, some silica-based glasses are not nearly as resistant to crystallization as eqn. (2) predicts. However, in at least some of these glasses the crystal growth is definitely catalysed by the taking up of impurities at the surface which may reduce 71 in the interfacial region. In common experience crystallization in undercooled liquids is almost always nucleated ‘heterogeneously’on seeds which are either present inadver- tently or deliberately injected into the system. These seeds may be crystals of the material itself or other solid materials such as the container walls or 478 David Turnbull
particles suspended in the liquid. At a given cooling rate the undercooling required for heterogeneous nucleation varies widely with the composition and structure of the seed material. Nucleation within the liquid and without the help of seeds is said to be 'homogeneous'. Experimentally it is difficult to circumvent the effects of seeds and thereby expose homogeneous nucleation behaviour. Liquids commonly contain lo5to lo6 suspended particles per cm3. When the liquid is undercooled, nucleation will occur first on the most effective seed and then, unless the crystal growth rate is very small, recalescence will cut off the possibility of any further indepen.dent nucleation either on other seeds or homogeneously. This is what occurred in Fahrenheit's experiments on the freezing of water. One of the most effective techniques(13)for revealing homogeneous nucleation is that of dividing the liquid into a number of droplets much greater than the number of suspended particles. Then, unless the process of subdivision and droplet isolation coated the droplets with a nucleating film, heterogeneous nucleation would not be possible in most of the droplets.(W Droplet experiments have shown that in some liquids homogeneous nuclea- tion is too infrequent at any undercooling to be measured. Such liquids, if they contain no seeds, can be easily cooled to the glass state without crystalliza- tion. Most other liquids, including monatomic ones, must be undercooled at least to T,= 0.85 to 0.67 before homogeneous nucleation becomes measur- able.(l4),(15), (l6) This resistance to the initiation of crystallization is extra- ordinary considering that in many of these liquids the average frequency with which an atom changes places with another is of the order of 1Ol1Hz, a frequency which is actually approached in the crystal growth process. It is perhaps the principal factor which sometimes makes possible the under- cooling of a liquid to the glass state. The crystal nucleation resistance of liquids can be understood in the frame- work of the simple nucleation theory(17)deriving from the contributions of Gibbs, Volmer and others. According to this theory the resistance arises from the work, uA, which must be done to form the crystal-liquid interface; A is the area and u is the tension of the interface. Nucleation is motivated Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 by the change in free energy in the crystallization of the bulk liquid; this is the product of the volume, v, of the crystal and the free energy change per unit volume:
where AH, is the molar heat of fusion, AC, is the molar difference in heat capacity between the crystal and the liquid and P is the molar volume of the crystal. The balancing of these two factors, one increasing with the area and the other decreasing with the volume of the crystal, leads to the condition that only those crystals containing more than some critical number, i", of molecules can, on the average, become nuclei for crystal growth; that is, crystals with fewer than i" molecules will, on the average, remelt. At small undercooling the critical number may be millions of molecules since, under these conditions, it increases as 1/(ATr)3. Under What Conditions can a Glass be Formed? 479
Thus the homogeneous formation of a crystal nucleus requires that a fluctua- tion in the liquid bringing together i" molecules in the crystalline configuration must somehow occur. Kinetic analysis indicates that these fluctuations most probably grow and decay by a series of steps in each of which one or a small group of molecules are added to or detached from the crystallite. Nearly all of these fluctuations will decay before reaching critical size. However, in one em3 of liquid with a viscosity of poise there is a measurable probability of a fluctuation which brings as many as one or two thousand molecules into a crystallite. Even a fluctuation of this magnitude might seem much larger than we would naively have considered possible. We can understand the large fluctuation by recalling that the average period for the attachment of a molecule to the crystal from the liquid is only about s and that a fluctuation decays by a path which is the reverse of that taken in its growth. This means that a fluctuation is no more likely to disintegrate in one co-ordinated set of move- ments than it is to form by the reverse of such a set.
3. Conditions for bypassing crystallization Consider now what conditions must be fulfilled by the crystal nucleation frequency and the cooling rate of the liquid if crystallization is to be bypassed. The actual number, Sn, of crystal nuclei which appear isothermally in a volume, v1, of liquid in time St is:
Sn = Ivl St (4) where I is the nucleation frequencyl(vo1ume x time). In a liquid with a low viscosity the crystal growth rate is so large that the cooling rate will be limited by the recalescence after a single nucleus has appeared. Under these con- ditions nucleation would have to be suppressed completely for crystallization to be bypassed. This means that n would have to be less than 1, where
n=q I dt, Jo t is the time in which the liquid is cooled, I is a function of temperature, and the variation of vl with temperature is neglected. These relations, (4)and (5), Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 indicate that the probability of forming a nucleus will be less the smaller are the volume of the liquid and the nucleation frequency and the greater is the cooling rate. Consider first the nucleation behaviour of a liquid, which has been entirely freed from heterogeneities, when it is undercooled from its thermodynamic crystallization temperature, Tm. Formation of the first nucleus will require at least the time, tmin, for all molecules constituting the nucleus to jump from the liquid to the nucleus,(l'J) i.e. tmin=i*71. At viscosity poise,
T( N 10-l2 s and, setting i* N 100, tmin N 10-lO s. Thus this liquid would not crystallize if it were cooled to its glass temperature in less than 10-lo s. This would require a cooling rate of the order of 10l2K s-l; such rates might be achieved in computer simulated experiments but they have not been realized in the laboratory. Under normal conditions the cooling periods are many orders of magnitude greater than tmin for low viscosity liquids and there is good reason for supposing that a steady distribution of sub-critical crystallites is always approached. 480 David Turnbull
A kinetic analysis based on simple nucleation theory with the additional assumptions that (a) 71 scales as the viscosity and (b) AC,=O in eqn. (3), leads to the following expression for the steady frequency of nucleus formation: kn I= -exp [-ba3p/Tr(ATr)2] 77 where k, is a constant specified by the model, b is a constant determined by the nucleus shape, a and /? are dimensionless parameters defined as:
where N is Avogadro's number. The principal resistance of a fluid to nucleation is due t'o a which is pro- portional t'o the liquid-crystal interfacial tension, u. Physically a is the number of monolayers/area of crystal which would be melted at Tm by an enthalphy (AH)equivalent in magnitude to u. The forms of the upper limiting nucleation frequency-reduced undercooling relation, with different assignments to a/?1/3,are plotted in fig. 2. To compute the numbers b was assigned its value for the sphere (16n-/3), 77 was set equal to lop2poise, independent of temperature, to give an upper bound for I, and k,
I I I I I I I I I Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010
I 1 I I I I I I 0.8 0.6 0.4 0.2 + Tr FIG. 2. Calculated variation of the logarithm of the frequency (in ~rn-~s-l)of homogeneous nucleation of crystals in an undercooled liquid with reduced temperature for various assignments of ~yp1/3. Under What Conditions can a Glass be Formed? 48 1
was given the value 1030dyn cm (loz3N m), which is approximately that obtained in the kinetic analysis of Fisher and the author.PO) We note that I,though rising steeply with ATr, is negligible at small under- cooling. In fact, to be observable within the liquid volumes and time periods which are practical experimentally, I must become larger than, e.g., ~m-~s-l(l m-3 s-l). This means that the part of the I- AT, relation closest to equilibrium, and where the simple theory is most valid, is practically inaccessible to experiment. With further increase in ATr, I increases to a broadly peaked maximum at Tr = 2~ and then falls to zero at 0 K. The curves in fig. 2 indicate that liquids with 01p1/~> 0.9 would practically not crystallize, unless seeded, at any undercooling. Thus they would form glasses if sufficiently undercooled. In contrast, it should be practically impossible to suppress, upon cooling to 0 K, the crystallization of fluids for which is small, e.g. < t. Experience indicates that /3 lies between 1 and 10 for most substances and is near one for most simple monatomic liquids, such as metals. 01 has been measured directly only in a few instances and there is no rigorous theory for predicting it. It is reasonable to think that it may be no greater than unity, i.e. one melted monolayer, and plausible arguments(21),(22). (23) have been made which would place it from to 4. The recent measurements of Glicksman and V~ld(~~%)give a - 0.4 for bismuth. Droplet nucleation studies indicate that 01 is at least 4 for many simple materials, including metals and alkali halides.(24) Since ,!? is near unity for these materials, a/31f354. The I - ATr relation (fig. 2) for afl1I3= 4 indicates that these liquids should resist crystal nucleation to a large undercooling; indeed, unless seeded, they should practically persist indefinitely when ATre0.18. At ATr=0*18, i" has been reduced to about 500 molecules. With further increase in AT, nucleation becomes copious and its frequency Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010
1 -301.0 oa 0.6 0.4 0.2 0
+Tr Fig 3. Variation of logarithm of frequency (in 0111-3 s-1) of homogeneous nucleation of crystals in liquids with reduced temperature calculated from eqn. (6). 01/3l/3 was set equal to 4 and viscosity was calculated from the Fulcher equation with indicated assignments of T,,,assumed equal to T,, in Fulcher equation. 2F C.P. 482 David Turnbull
should broadly peak at about cm4 s-l. Even at the fastest cooling rates achieved in the laboratory, !!' - - lo7K s-l, crystallization of these liquids would be suppressed (i.e. n < 1 in eqn. (4)) only if w1< om3, i.e. in liquid drops containing no more than 1000 molecules. Under more normal con- ditions, e.g. - - 1K s-l, w1- 1 cm3, it would be impossible to chill any liquid in this class to a glass, supposing that its Tgwere not much greater than 0 K. If, as we are supposing, I and u both scale, at given AT,, as 117, then the glass-forming tendency should increase with the reduced glass temperature, Tg/Tm=Trg. The effect of different assignments of Trg on the nucleation frequency, calculated from the simple theory, with cu/3113 = 4, is shown in fig. 3. The viscosity was calculated from an equation of the Fulcher form with, e.g., constants typical of a number of simple molecular liquids and with Tr, = Trg. In particular, 7 was equated to lop3' exp [3*34/(T, - Tg)]poise. We see that the effect of increasing Trg is to lower, sharpen and shift, to higher Tr, the peak in the I - Tr relation. Liquids with glass temperatures as high as QTm,if seed-free, would practically crystallize only within a narrow temperature range and then only slowly; thus they easily could be undercooled to the glass state. Liquids with glass temperature TmlZ could, according to these relations, be chilled to the glass state only in relatively small volumes and at high cooling rates. For example, at T- - los K s-l they would form glasses provided w1< cm3, i.e. in droplets with diameter less than 60 pm. Of course the value of Tr, required to form a glass at a given cooling rate will be lower the higher is cu/31/3. The preceding analysis has focused on the conditions required for bypassing crystal nucleation to the undercooling required for reaching the glass state. These are the most important conditions for forming glasses from liquids which are quite fluid at their melting temperature. Now consider the conditions required to form glasses from liquids which already contain crystal nuclei at Tm either from impurity particles or added seeds. These are the conditions which normally would be critical for com- mercial glass formation since it is extremely difficult to eliminate all nucleating heterogeneities from large volumes of material. Clearly they will be deter-
Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 mined largely by the crystal growth rate, u. When 19 AT,,, where AT,, is the reduced undercooling at the liquid- crystal interface, u can be expressed by(17),(l9:
where X is the distance the interface moves in each interfacial jump and f is the fraction of sites in the crystal surface to which molecules can be attached; e.g. in a step mechanism for growth f is the fraction of surface sites which lie at step edges. Supposing, as before, that - lO-l07, eqn. (7) becomes
We have noted that ATq is smaller than the reduced undercooling AT,,, at the edge of the liquid owing to the limited rate of transport of heat of crystalliza- tion away from the interface. The limiting effect of heat flow on the crystal growth rate can be shown with the following oversimplified model. Suppose Under What Conditions can a Glass be Formed? 483
that heat is transported at a steady rate over a distance 6 to a sink at reduced temperature Tro and the interface remains relatively flat during growth. Then u becomes lolo Apf ATr, u= -- 7 1+K where
and K is the thermal conductivity of the liquid, u [ - ( P/pR)(~hTr,/6)]is governed mainly by the rate of transport of heat from the interface; this is also the condition for marked recalescence and the crystallization of the liquid by a single nucleus. In contrast when 1 BK, AT,, =ATIo and u [ - (lO1OApf/~)ATro]is controlled mainly by molecular processes at the interface. For most materials ( A/I2/P)- 3 x and since R = 8.3 J mol-l K-1 K-2x 10g(f6/~7) (8b) to within an order of magnitude. With f = 1 and 6 = 0.1 cm, the condition that 1 > K is 7 > 10 poise for a metallic liquid (K - lo7) and r] > lo3poise for a non-metallic liquid (K - lo2J m-1 s-l K-l= lo5erg cm-l s-l K-l). Actually, for most molecular liquids, especially the glass formers, f is much less than unity near T, so that the condition for 1 > K would be closer to r] > 1 poise. Accord- ing to eqn. (8) the maximum crystal growth rate at the viscosity 1015poise, which delineates fluid from solid behaviour, would be about 10 nm per day. Given the relations which have been presented the calculation of the con- ditions for glass formation would be a straightforward, though tedious, procedure. However, certain limiting conditions can be easily deduced. For example, consider a non-metallic liquid in which the average seed spacing is 0.1 cm. Such a liquid could be chilled to a glass at cooling rates easily achieved in the laboratory providing its viscosity at the melting point was 103 poise or greater. Fused silica has a viscosity of about 1O7 poise at the melting point of cristobalite; with an average seed spacing of 0.1 cm it ought to form a glass Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 even when cooled at rates as low as 1 to 10 K/hr. Thus whether or not the crystallization of an undercooled liquid is bypassed will depend principally upon a set of factors which can be controlled to some extent in the laboratory, namely, the cooling rate, - 2h, the liquid volume, v1, and the seed density, ps, and upon a set of materials constants, namely, the reduced liquid-solid interfacial tension, a, the fraction of acceptor sites, f, in the crystal surface and the viscosity-reduced temperature relation which often can be characterized roughly by the reduced glass temperature, Trg. The glass-forming tendency will be greater the larger are -$, a and Trg and the smaller are vl, ps and f.
4. Effect of molecular constitution on glassforming tendency Consider now the variation of 01, f and Trg among different types of sub- stances. Though a appears to be 7 + for many simple substances, experience is hardly sufficient to generalize about its variation with material type. Thus 484 David Turnbull
it is possible that the ease with which some substances form glasses is due mainly to their large 01 (e.g. a> 0.8 to 0.9). The fraction, f, of acceptor sites in the crystal surface can be very small when the equilibrium form of the crystal-liquid interface is molecularly smooth; in this event the interface can only advance by the lateral movement of molecular steps formed by two-dimensional nucleation or by the emergence of screw dislocations in the surface. Jackson(25) discovered that the step mechanism of crystal growth apparently predominates in those materials in which /3 (i.e. ASmJR)$2. This is demonstrated by the predominance of poly- hedral, as opposed to dendritic, morphologies in the growth of single crystals of these materials. It is likely that growth rate limitation by the step mechanism is partly responsible for the ease with which liquids such as glycerol (/3=7.5), salol (/3=7) and boron oxide (18-3.5) undercool to glasses. Boron oxide seed crystals, even after partial melting, have not been observed to grow perceptibly at atmospheric pressure in liquid boron oxide at any undercooling. This may reflect that the acceptor site fraction of boron oxide is extremely ~ma11.(25~) Various proposals have been made for correlating glass temperatures. These include that Tg scales as Tm,as the cohesive energy(lO)or as the Debye temperature.(26) Ka~zman(~')noted that Tr, is approximately $ for a number of simple molecular substances which easily form glasses in bulk. However, our analysis indicates that Tr, must lie considerably below Q for those sub- stances which do not form glasses in common experience. Also Myers and Felty(28)found that in certain binary chalcogenide systems T, does not scale as the liquidus temperature but rather goes smoothly with composition between the glass temperatures of the pure components. Cohen and the author(lO)pointed out that the simple models for liquid viscosity suggest that T, should scale roughly as cohesive energy, which can be approximately indexed by the normal boiling temperature, Tb, for sub- stances of a given type. This correlation is consistent with the effect of molecular asymmetry on glass-forming tendency. In particular, it is well known that on the cohesive energy scale Tm is much lower for asymmetric
Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 than for symmetric isomers. This means that if T, is fixed on the same scale, the relative undercooling required to reach T, will be much less for asymmetric than for symmetric compounds. Indeed, it is commonly observed that liquids composed of asymmetric molecules form glasses much more readily than do those consisting of symmetric molecules. For example, consider the meta and para isomers of xylene. The two compounds normally boil within 1.5 K of each other and their viscosity- temperature relations are almost identical over the temperature range 10°C to 135°C (0.7 to 1 T,,). However, relative to Tb, the melting temperature of the meta compound is only 0-53 compared with 0.70 for the more symmetric para compound. The author was able to quench the meta, but not the para, compound in the form of 1 mm diameter droplets to the glass state. The cohesive energy correlation also provides some guidance for interpreting the effect of composition on the glass-forming tendency of solutions. For example, suppose that two substances with approximately equal cohesive energies mix according to a simple eutectic phase equilibrium diagram. Then T,, if it scales as cohesive energy, would be roughly independent of composition Under What Conditions can a Glass be Formed? 485
and the reduced undercooling required to achieve the glass state would be least at the eutectic composition.(29) Indeed, it has often been noted that the glass- forming tendency in binary eutectic systems is greatest near the eutectic compositions. It is noteworthy that the compositions of the only metallic systems, Au,S~(~O),(a1) and Pd,SiP), (33), found so far to exhibit the glass transition, thermally and rheologically, are near very deep-lying eutectics.
5. Special methods for forming glasses The considerations just presented are useful for deciding which materials are most likely to form glasses. Now consider the problem, given a material, what techniques can be used to put it into the glass form. We have noted that if the initial form of the material is liquid, we are more likely to get a glass the finer is the state of dispersion of the liquid (i.e. the smaller is v1) and the greater is the cooling rate, -Ih. To achieve the maximum cooling rates, the liquid should be spread as thinly as possible over the surface of a good thermal conductor held at a low temperature. The most effective techniques for doing this appear to be the hammer and anvil method and the ‘splat’ cooling method developed by Duwez and collaborators(30),P2), (34). In the ‘splat’ method a liquid droplet is driven at a high velocity on to the spinning surface of a cold metal such as copper. In this way the droplet is spread out rapidly into a very thin film, e.g. 1 pm thick, in good thermal contact with the metal. In the hammer and anvil technique, two cold metal blocks are clapped together as liquid is dropped between them. With these techniques metallic
liquids can be cooled at rates as high as N lO7Ks-1. Using the ‘splat’ method Klement, Willens and D~wez(~O)demonstrated, apparently for the first time, that a metallic glass, an alloy with approximate composition Au,Si, can be formed directly from its own melt. To this point we have considered principally the formation of glasses directly from their own melts. Alternatively glasses, or ‘amorphous solids’ for those preferring the technologists definition of glass, often can be formed by con- densation from solutions, gaseous, liquid or solid. The condensation must be made to occur on surfaces held well below the glass temperature of the con- densing material. It may be effected by vapour, electrolytic or chemical Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 deposition or by phase separation. Usually the rates of the deposition processes can be limited sufficiently so that the heat of condensation does not heat the condensate above T,. Application of these methods has greatly enlarged the number and variety of materials which have been put into the glass form. For example, Pryde and J0nes,(3~)by condensing vapour on to a cold (77 K) substrate, obtained an apparently glassy form of water, although no one seems yet to have succeeded in quenching liquid water to a glass. Also the most stable liquid form of elemental germanium, which is metallic, has not been quenched to the glassy state. However, germanium has been put into an amorphous solid semiconducting form by condensation of germanium vap~ur(~~&),by electrolysis(36b) of a solution of GeC1, from anhydrous propylene glycol and, more recently, by phase separation of solutions of germanium in germanium oxide.(37) A large number of metallic alloys have now been made in solid amorphous form by the various deposition techniques. These include alloys of the 486 David Turnbull
transition metals, Ni, Co and Fe, with phosphorus or sulphur and of copper and silver.(38) For example, amorphous alloys in the composition range Ni,P to Ni3P have been made by vap~ur,(~~)electro and chemical deposition.(40) The radial distribution functions of the electrodeposited amorphous alloyd8) are very similar to those of the splat-formed alloy glasses.
6. Do all materials form glasses? We now return to the question: Can all materials be put into the glass form? Experience shows that, if we categorize solids by bond type, covalent, ionic, metallic, van der Waals or hydrogen, there are at least some glass- formers in every category, although the number of ionic(41)or metallic glasses found so far is quite small. However, the most convincing evidence for the universality of the glass state would be the demonstration that puremonatomic substances, such asargon or gold, can be put into the glass form. Hope that such a demonstration might be possible stems from the model studies of Bernal(') and of Cohen and the author,(lO)which indicate that a monatomic glass may be internally stable, and the experience that metallic alloys in which one component predominates, e.g. Au,Si, Pd,Si, Ni4P, can be put into the glass form. It would seem that the technique most likely to produce a monatomic glass is deposition from the vapour on to a very cold surface. SO far as the author knows, no one has succeeded in depositing a glass of any pure monatomic substance having as its most stable form a crystal with a close-packed structure. Hilsch and collaborators(42)made extensive studies of the structure and annealing behaviour of metal films deposited on surfaces cooled by liquid helium. Of the pure metals only the films formed from bismuth or gallium proved to be amorphous and these crystallized at a very low temperature, -20 K. However, small impurity admixtures, e.g. Cu with Sn or SiO with copper, did result in the formation of films which were amorphous and remained so during annealing to room temperature. That bismuth and gallium films crystallized rapidly at low temperatures indicates that crystal growth rates into amorphous pure metals may be many orders of magnitude greater than calculated from the viscosity correlation. The failure in most instances of Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 Hilsch and associates to obtain pure amorphous metal films may reflect these high growth rates and the presence of nucleating heterogeneities on the sub- strate. Also, the amorphous films if formed, might not have been in their lowest energy states, owing to relatively rapid rates of deposition. The conditions which would seem most favourable for the formation of pure amor- phous monatomic films are the slowest possible deposition rates on to substrates entirely freed of crystallites. It seems established that some impurity admixtures favour formation of metallic glass. One possible mechanism for this effect, a decrease in reduced undercooling at the glass temperature due to depression of Tm by impurity addition, has already been discussed. Alternatively Nowick and Mader(38) have suggested that metallic glass solutions may be stabilized primarily by large disparities between the sizes of the constituent atoms. However, we note that differences between the metallic atom radii, rmo, in the pure metallic state, of the constituents of some of the most stable metallic glasses are quite small. For example, for Au,Si, rA,,"= 1-44 a = 0.144 nm; r,io = 0.142 nm; for Under What Conditions can a Glass be Formed? 487
Pd,Si, rpdo= 0.138 nm; and for Ni,P, rN: = 0.124 nm, rpo=0.128 nm. Of course, the metallic radii may change substantially in the mixing process, though no significant changes in Ni,P are indicated by density measurements on electro- deposited amorphous films.(*). Apart from their effect on the forces driving crystallization, dissolved impurities may markedly reduce the kinetic coefficients of crystallization. For example, impurities may necessitate atomic rearrangements, such as local ordering or segregation, in the crystallization process. These rearrangements are much more likely to be limited by processes similar to those which impede viscous Aow than are the interface processes in the crystallization of pure monatomic substances. Thus eqn. (7a) may sometimes be a fair approximation for the rate of growth of crystals into an alloy glass. The most stable homogeneous states of the glass-forming alloys Au,Si and Pd,Si, are ordered intermetallic phases characterized by very large unit cells. Professor Mott suggested that the resistance of the glass states of these alloys to crystallization may be due in part to difficulty in nucleating crystals in which the cells are so large. However, the question arises why don’t these alloys crystallize to completely disordered close-packed solutions? Presumably the driving free energy for this process must be very small at the glass tempera- ture of the alloys. In this connection the mode of decomposition of palladium-silicon glasses is interesting.(33),(34). When heated to just below their glass temperature, they crystallize to a homogeneous face-centred-cubic state which exhibits no long- range order. However, when heated above their glass temperature, in certain composition ranges, they separate into two phases, both amorphous initially, with morphologies very similar to those of phase separated soda-lime silicate glasses.
REFERENCES (1) FULCHER,G. S., 1925, J. Am. Ceram. SOC.,6, 339. (2) (a) HETHERINGTON,G., JACK,K. H., and KENNEDY,J. C., 1964, Phys. and Chem. of Ghses, 5, 130; (b) FONTANA,E. H., and PLUMMER,W. A., 1966, Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 Phys. and Chem. of Glasses, 7, 139. (3) COHEN,M. H., and TURNBULL,D., 1959, J. Chem. Phys., 31, 1164. (4) ZACHARIASEN,W. H., 1932, J. Am. Chem. SOC.,54, 3841. (5) BELL,R. J., and DEAN,P., 1968, Phys. and Chem. of Glasses, 9, 125. (6) GRIGORVICI, R., 1968, Mat. Res. Bull., 3, 13. (7) BERNAL,J. D., 1959, Nature, 183, 141; 1960, 185, 68; BERNAL,J. D., and MASON,J., ibid., 1960, 188, 910. (8) CARGILL, G. S., 111, 1969, Ph.D. Thesis, Harvard University, Cambridge, Mass. (9) GIBBS, J. H., and DIMARZIO,E. A., 1958, J. Chem. Phys., 28, 373. (10) COHEN,M. H., and TURNBULL,D., 1960, (a)J. Chem. Phys., 34,120; (b) Nature, 1964,203, 964. (11) TURNBULL,D., 1962, J. Phys. Chem., 66, 609. (12) WALKER,J. L.; results are published in B. CHALMERS’PrincipZes of SolidiJication (New York: Wiley, 1964), pp. 114-115. (13) VONNEUUT,B., 1948, J. Colloid. Sci 3, 563. (14) TURNBULL,D., 1949, J. AppZ. Phys., 20, 817; 1950,21, 1022. (15) THOMAS,D. G., and STAVELY,L. A. K., 1952, J.Chem. SOC.,4569; DENORDWALL, H. J., and STATELY,L. A. K., 1954, ibid., 224. (16) For review see TURNBULL,D., 1961, Trans. AIME, 221, 422. 488 Under What Conditions can a Glass be Formed?
(17) For review see (a) TURNBULL,D., 1956, Solid State Physics, 3 (New York: Academic Press), pp. 226-309; (b) CHRISTIAN, J. W., 1965, Theoryof Phase Transformations in Metals and Alloys (London: Pergamon). (18) CAHN,J. W., HILLIG,W. B., and SEARS,G. W., 1964, Acta Met., 12, 1421. (19) HILLIG,W. B., private communication. (20) TURNBULL,D., and FISHER,J. C., 1949, J. Chem. Phys., 17, 71. (21) SKAPSKI,A. S., 1956, Acta Met., 4, 576. (22) CHALMERS,B., 1959, Physical Metallurgy (New York: Wiley), pp. 244-245. (23) TURNBULL,D., 1965, (a) Liquids: etc. (edited by T. J. Hughel) (Amsterdam: Elsevier), pp. 6.25; (b) 1965, Physics of Non-Crystalline Solids (edited by J. W. Prins) (Amsterdam: North-Holland), pp. 41-57. (23a) GLICKSMAN,M. E., and VOLD, C. L., 1969, Actu Met., 17, 1. (24) BUCKLE,E. R., and UBBELOHDE,A. R., 1960, Proc. Roy. SOC.,259 A, 325. (25) JACKSON,K. A., 1958, Growth and Perfection of Crystals (edited by Doremus et al.) (New York: Wiley), pp. 319-324. (25a) UHLMANN,D. R., HAYS,J. F., and TURNBULL,D. 1967, Phys. and Chem. of Glasses, 8, 1. (26) ANGELL,C. A., private communication. (27) KAUZMAN,W., 1948, Chum. Rev., 43,219. (28) MYERS,M. B., and FELTY,E. J., 1967, Mat. Res. Bull., 2,535; see also Rochester, New York, Conference on ‘Characterization of Materials’, November 1967. (29) COHEN,M. H., TURNBUL BULL, D., 1961, Nature, 189, 131. (30) KLEMENT,W., WILLENS,R. H., and DUWEZ,POL, 1960, Nature, 187, 869. (31) CHEN, H. S., and TURNBULL,D., 1968, J.Chem. Phys., 48,2560; 1967, J. Appl. Phys., 38, 3646. (32) DUWEZ,POL, WILLENS, R. H., and CREWDSON,R. C., 1965, J. Appl. Phys., 36, 2267. (33) CHEN, H. S., and TURNBULL,D., 1969, Acta Met. (in press). (34) For review of rapid cooling techniques and results of DUWEZand associates see DUWEZ,POL, 1967, Trans. ASM, 60, 605-633. (35) (a)PRYDE, J. A., and JONES,G. O., 1952, Nature, 170, 685; (b) see also ANGELL, C. A., SARE,E. J., and BRESSEL,R. D., 1967, J. Phys. Chem., 71, 2759. (36) (a) RICHTER,H., and BREITLING,G., 1958, 2. Naturforschung, 13a, 988; (b) SZEKELY,G., 1951, J. Electrochem. Soc., 98, 318. (37) DE NEUFVILLE,J. P., 1969, Ph.D. Thesis, Harvard University, Cambridge, Mass. (38) (a) NOWICK,A. S., and MADER,S. R., 1965, IBM Journal of Res. and Develop- ment, 9, 358; (b) see also M~DER,s. R., WIDMER,H., D’HEURLE,F. M., and NOWICK, A. S., 1963, Appl. Phys. Letters, 3, 201.
Downloaded By: [Fyzikalni ustav AV CR] At: 06:09 11 August 2010 (39) (a) BAGLEY,B. G., 1967, Ph.D. Thesis, Harvard University, Cambridge, Mass.; (b) BAGLEY,B. G., and TURNBULL,D., 1968, J. AppE. Phys., 39, 568. (40) (a) BRENNER,A., COUCH,D. E., and WILLIAMS,E. K., 1950, J. Res. Nut. Bur. Standards, 44,109; (b) BRENNER,A., and RIDDELL,G., 1947, J. Res. Nut. Bur. Standards, 39, 385. (41) For review see ANGELL,C. A., 1966, J. Phys. Chem., 70, 2793. (42) HILSCH,R., 1960, Non-CrystaEEine Solids (edited by V. D. Frbchette) (New York: Wiley), pp. 348-373. (43) POLK,D. E., 1920, Ph.D. Thesis, Harvard University, Cambridge, Mass. Glasses and its history
Jaroslav Šesták
Some introductory aspects of glass and its historical features
Glass is often viewed as a remarkable non-crystalline substance regarded to have magical origin though usually made from the simplest raw materials upon the effect of firing. Mimicking evolution, however, became responsible for the creation of further families of a wide variety of glassy materials, which have gradually appeared through human creativity particularly during last hundred years. Properly chosen procedure of rapid extraction of heat (= quenching) turned up to be in charge for a successful glass-formation (= vitrification) of almost all substances in general (thus assistant for physicists to prepare glassy state from different sorts of various inorganic materials even metals) in contrast to the previously mentioned, traditional chemical approach seeking just for an appropriate composition to vitrify under a customary self- cooling.
The first natural glasses were formed as the Earth cooled and therefore predate creation of living organisms by about 1.5 billion years. Such primordial glasses were limited in composition and versatility as were the first primitive unicellular organisms (bacteria), however, some compositions (in an unstable state of glass) have survived unchanged enormously long (similarly to certain strains of bacteria). A still open intellectual terrain ahead is apparent in each of the material class associated with real bodies either advanced naturally or created synthetically - particularly interested in glasses when assuming rather curious but apparent degree of similarity with biopolymers (genes) [85]. Both are in a reinforced (unstable thermodynamic) state formed due to kinetic reasons, which are existing until glasses transform into a stable structure (melt of the lowest free energy) or biopolymers (occurring as parts of living organisms) stops its biological activity (life). Both glasses and biopolymers adapt to their environment and previous history and there is an almost limitless range of compositions of glasses and countless numbers of different living species, each family having particular molecular building blocks and severe structural rules although some compositional restrictions of glasses were gradually overcome. Although the origin of glass is nevertheless lost in prehistorically obscurity all imaginable glasses can be synthesized making thus the natural and manmade glasses indistinguishable. Another intriguing parallel can be drawn in their spontaneously apparent emergence of new phyla or groups of species involving both the disordered state of glasses and highly ordered situation of biopolymers as well as their often highly non-equilibrated status.
The structure and properties of these materials are understood and subjected to monitor in ways that were unheard of decades ago (although materials have been central to the growth, prosperity, security and quality of life of humans since the beginning of history [86-93]). Despite all the similarities one difference is fundamental: life forms are self- replicable and living organisms cannot be created in the laboratory yet; any attempt did not even produce a simple protein macromolecule. All cells contain proteins and, on the other hand, proteins synthesis requires enzymes. So how biosynthesis could begin without enzymes is the question to be resolved. This dilemma may involve our better understanding of possible convergence of glass and genes assuming known bioactivity of certain inorganic glasses, particularly their “–OH” active surface capable of binding to living tissues under their specific compositions, porosity (‘self-similarity’ and ‘fractality’ [2,3,12]) and pre-treatments [2,85]. The diversification of glasses has been known since humans learned how to control fire - roughly few hundred thousand years ago. The first manmade glasses were synthesized unintentionally by the fortuitous smelting of sand and alkaline plant flux by fire about ten thousand years ago. Glass-working was mentioned with connotation “just to take plentiful sand and plant ashes and, by submitting them to the transmuting agencies of fire to produce melt which whilst cooling could be shaped into a an infinite variety of forms, which would solidify into a transparent material with appearance of solid water and which is smooth and cool to the touch, was and still is a magic of the glass workers art ”. Some glasses were created naturally [86] by accidental melting due to the charge of lightening (‘lechatelierite’) and worth of noting are ‘obsidians’ (glassy volcanic rocks consisting of natural acidic silicate glasses exhibiting a high melting point) and ‘pumice’ (famed volcanic glass), which were formed by terrestrial upshot of volcanism and have attracted the men’s attention since prehistoric times (used as cutting instruments, amulets or cult objects). A special attention deserves tektite-strewn fields famous for deeply sculptured pieces of olive greenish, usually rapidly frozen, droplets of a meteoritic impact origin (in the Southern Bohemia are known as Moldavites since their proper description in 1787 when they were listed among precious stones used in jewelry). It is also worth noting that the extent of natural glass on earth is in the range of a tenth of a percent with the ratio of about 4000 minerals versus mere 5 types of natural glass while on the moon (and possibly also other planets) it is possible to identify only ~60 minerals against 35 glasses- the frequency of glass deposits being at least one order of magnitude higher than that on the Earth.
An important role in the medieval development of glass making was played by North Bohemia [90,91]. Surrounded by mountains the most remarkable factor was the creation of the oldest glassworks in the continent, some of them were operating without interruption from its foundation (such as in little town Chřibská since 1414). Contrary to the greater part of preserved German medieval glass, usually of a rich green color due to the admixture of iron, the glass of Bohemian firings [92,93] is as a rule lighter, almost transparent, and only a little tinged with yellow, brown or light green. Glass cups and goblets of various shapes were decorated on their surface with large or small stuck-on bead adornments. Hollow glass of the 14th Century manifested a long-standing production tradition with its refined shapes and difficult and complicated hand-executed decorations. The most typical glass of the Czech environment were flute shaped glass beakers, tall slender goblets called the "Bohemian type", Bohemian glass, however, was never so fine and flexible as so called “Venetian glass” [90], which was more suitable for forming fantastically shaped vessels (because rich in soda from sea plants it was less viscous). On the other hand, beech (and both less frequent and suitable oak) wood used for firing, the refined ash, rich in lime and potash, and the country's quality siliceous sand contributed to the greater purity of the Bohemian glass which was composed of potassium calcium silicate. Hard, clear glass which resembled natural rock crystal, generally called Bohemian mountain glass, was suitable for engraving and foreign producers considered themselves privileged when given the right to make glass marked "ad uso de Bohemia".
Several notable milestones of more modern science depended on the availability of onwards glass as a preeminent choice of alchemist for their apparatuses. Relatively unstable and fragile glass was always essential for many chemical operations in early times. Dissatisfied with the chemical durability of glass, glass properties were modified by adjusting composition; however, this could not yet have been done properly because chemistry was still on a mystical basis, and the techniques of analysis needed did not exist. An important cornerstone was Galileo's work on the motion of planets based on glass lenses in astronomical telescopes as well as Newton's pioneering work in optics (1666) requiring in addition prisms and mirrors. Other basic investigations required specific glass apparatuses to describe the properties of gases, introduce thermometry, barometry and develop microscopy. The first reasonably documented description of glass-making procedures is associated with the invention of lead glass around 1676. The most influential books appeared to be Neril's "L'Arte Vetraria" (1612) and Kuncel's "Ars Vitraria Experimentalis" (1679) which were translated into other languages (as well as many others such as the Encyclopedia published in the year 1765, etc.) all of them remained, however, not more than recipe manual. In the middle of the seventeenth century a proper understanding of heat was yet lacking so that melting, solidification and glass formation could not be understood well enough so that the only important glass properties were those easily measurable were, such as density and refractive index.
The other problem was associated with furnaces, first, the material to build them from, secondly, how to attain a high enough temperature together with a way to measure and control it. The net caloric value of wood decreased with moisture content, falling almost to zero at about 70% of moisture. Even well dried wood still contained around 20% of moisture so that the maximum temperature attainable did not exceed 1200 oC. A typical (Bohemian) furnace was above ground level, vaulted and oval in shape, in which glass was obviously also pre-melted, melted and blown, with a lower part for annealing of glass on the opposite side to the orifice for product insertion. The exhausted gases passed over the pots and out of the working holes where a space was reserved for oak wood drying. An extraordinary good level of medieval glassmaking is exhibited by the quality of mosaic still retained on the Golden Gate of Prague's St. Vitus Cathedral ("The Last Court" from 1370-1376).
Since medieval times manual skill allowed the making of window glass by the crown process (forming a shallow bowl and, after reheating, spinning to make it open into almost a flat circular disc) and by the cylinder process (blowing a cylinder, cutting off the ends and cracking it longitudinally) often mentioned as "procede de Boheme". Chemically resistant boron-containing glass was produced as early as in 1830 by Kavalier and later continued by high silica glass (80% SiO2) called “unexcelled”. The nineteenth century showed an enormous progress in optical glasses through effective stirring and later by fruitful investigation of the property versus composition relationships. The tank furnace made possible continuous large scale production, and machines for automated production of containers were most innovative in the glass industry. By the turn of the century Owens produced a successful six arm rotary machine which differed from most others in the way it was supplied with glass, and it remained almost unchanged until the 1960s when it was supplanted by gob feeders. Patents for sheet glass production date back to the 1850s but with little success. The invention of mechanical drawing of sheet glass was made by the Belgian Fourcault in 1914 but not finalized due to the war. The actual production took place in 1919 in a newly built glassworks in North Bohemian Hostomice. The original serial pulling was there later improved by the system called "Bohemian cross", but the most important large scale production took place in the 1950s by a rather expensive redundant plate process (laying melt to cool on the surface of liquid tin). The production of foam glass and fused basalts is also worth mentioning in the same period.
The large scale production of glass fibers for insulation and for textiles was another important twentieth century development as well as less known advances in high quality, dimensionally accurate pressing or optical fibers, gradient index glasses, etc. As a curiosity we can mention that the first British TV employed a screen in which bulbs were handmade by precise blowing from a single piece of lead glass and then were imported from the former Czechoslovakia in the late 1930’s. Since that the sphere of glass developed to unimaginable diversities, see next Fig. 11.
Fig 11. Various images of glass and glassy state, from left: cut led-containing oxide glass, magnetic metallic glassy ribbons, hydrophilic gelled lenses and micro-photos of either the biological glass emerging in undercooled plants and the non-crystalline portrayal of natural sedimentary opal.
As a matter of interest, Czech scientists have become world famous in macromolecular chemistry of polymers especially when introducing ‘hydro gels’ as a widely used material for the fabrication of contact lenses for eyes. Such an idea for correcting eye astigmatism was first mentioned in the year 1827 by the British physicist Herschel but was realized in practice as late as 1887 by the eye glass craftsman Müller who used very well cut and polished glass pieces. The Czech physician Teissler succeeded in replacing rigid oxide glass by a rubbery material based on celluloid foil as early as 1935. The major ‘mismatching’ disadvantage (the difference between the mechanical properties of lenses and the cornea) was not defeated until the 1952 invention by O. Wichterle and D. Lim who proposed the use of hydrophilic gels to match not only the mechanical properties of the cornea but also to enable a free exchange of biological liquids and oxygen. It was first prepared by radical polymerization of methacrylesters of ethylene glycol obtaining poly(2-hydroxyethyl methacrylate), abbreviated as poly(HEMA). It becomes easily water-swollen but still remaining mechanically applicable. The problem of preparation of optically well-defined lenses’ geometry was overcome by rotatory casting (legendary realized on bases of a toy brick-box) and since it has become a Czech patent widely applied in various countries (particularly USA). Nowadays this type of material is used not only for intraocular lenses but also for bio-imitation of other organs such as blood vessels, etc. It is clear that a special world of glassiness may naturally include constitution of almost all organic macromolecules, including polymers and overall plastics, which we herewith incorporated into our account of amorphous materials.
Following lines of glass investigation
The sort of non-crystalline materials, which are called glasses are special resources that are formed merely under favorable thermally non-equilibrium conditions; which are either provided by nature or created by man. We should note that the previously used term “non-crystalline” comprise two expressions where “amorphous” is a more general, generic term (often associated with methods of disordering such as intensive polycrystalline milling) in comparison with the term “glass” (linked with the melt cooling technique called ‘vitrification’); glasses subsists also a rather universal engineering material. Most glass products manufactured on a commercial scale are made by suitable cooling a mixture from their state of melt/liquid. For some particular applications, glasses are also made by other technologies, for example by chemical vapor deposition to achieve extreme purity, as required in optical fibers for communication, or by roller shilling in the case of amorphous metals, which need extremely high cooling rates (‘quenching’). Variation of the composition results in a huge variety of glass types, families, or groups, and a corresponding variety of properties. In large compositional areas of oxides, the properties depend continuously on composition, thus allowing one to design a set of properties to fit a specific application. In narrow ranges, the properties depend linearly on composition; in wide ranges, nonlinearity and step-function behavior have to be considered. The most important historical and subsequently engineering glasses are mixtures of various oxide compounds. For some special requirements, e.g., a particular optical transmission window or coloration, fluorides, chalcogenides, and colloidal (metal or semiconductor) components are also used. A very special glass is single-component silica, which is a technological material with extraordinary properties and many important applications. On a quasi-macroscopic scale (> 100 nm), glasses seem to be homogeneous and isotropic; this means that all structural effects are, by definition, seen only as average properties. This is a consequence of the manufacturing process associated with various cooling rates and undercooling states. A structure with a well-defined short-range order (on a scale of < 0,5 nm, to fulfill the energy-driven bonding requirements of structural elements made up of specific atoms) and a highly disturbed long-range order (on a scale > 3 nm, disturbed by misconnecting lattice defects and admixture of different structural elements). When on cooling the crystallization is bypassed, we speak of a frozen-in, super-cooled or better undercooled, liquid-like structure. This type of quasistatic solid structure is not controlled by traditional thermodynamics only but is greatly depending on kinetics and thus is not in a completely stable state, which tends to relax and slowly approach an associated equilibrium structure of melt (whatever this may be in a complex multicomponent composition representing a minimum of the Gibbs free energy). This also means that all properties change with time and temperature, but in most cases at an extremely low rate, which cannot be observed under the conditions of classical applications (in the range of ppm, ppb, or ppt per year at room temperature). However, if the material is exposed to a higher temperature during its practical dispensing or in the final application, the resulting relaxation may outcome in an objectionable deformation or internal stresses that then limit its consequent use.
The melt-cooling processes responsible for the formation of vitreous states are not the only methods which generate glassiness. Low temperature digenetic and biotic processes also result in amorphous solids such as hyalites and opals (when the ‘opaline’ state is a status containing curiously a low degree of short range order while attaining a high degree of order at long range). Other non-crystalline materials are various alumina-silicates (based on the mixture of meta-kaolin with an alkaline activator producing thus hardening pastes), which are frequently termed as alkaline inorganic polymers, geo-polymers or hydro-ceramics exhibiting, among other interesting properties, amalgamated qualities peculiar to cements with those of glass-ceramics and zeolites. They need a special attention targeted in the consequent chapters.
There are thousands worth noting researches, scientists and engineers who contributed better understanding of glass science and technology mentioning just few. Among the mostly significant scientific achievements was the Griffith's theory [94] of the strength of brittle materials published in 1920s. X-ray diffraction analysis was a particularly exciting field having enormous impact on glass science in the first quarter of the 20’s Century. It led Zachariesen (1932) to consider his principles on how bonding requirements were met and nearest neighbor coordination maintained without imposing an exact long range order [95] common for crystalline materials. Other important notional impacts were done by Tammann [96] , Vogel [97], Fulcher [98], Kauzmann [99], Tool [100], DiMarzio [102], Turnbull [102], Frenkel [106] (and many others such as Davis, Gibbs , Cohen, Angel, Fisher, etc.) not forgetting Kreidl [115], see Fig. 12., which were mostly aimed toward the theory of glass formation and glass transition. In particular Davies’ and Jones published early treaties on thermodynamics of glass formation [104] and from that time there have appeared countless studies, which were trying to clarify the disequilibrium state of glasses, the status of which, however, still remains in the focal point of better understanding of glassiness [105], see more details in this book introductory chapter. There is available a wide range of various books on inorganic [e.g. 106-140] as well as organic [141- 149] glasses and many thousands of papers laying outside of any real reach of citing and reviewing them (just mentioning our survey publications completed in the framework of the Institute of Physics [150-163]).
Fig. From left to right: Gustav H.J. Tammann (1861-1938) called earliest attention to a tendency, which revealed observation that that the higher the melt viscosity at the melting temperature, the lower is its crystallizability. He also invented the term „thermal analysis“. Frederik F.H. Zachariasen (1906-1979) considered the principles how the bonding requirements are met and the nearest neighbor coordination maintained without imposing an exact long range order. Walter Kauzmann (1916-2009) analyzed the nature of the glassy state and the behavior of liquids at lower temperatures, pointing out that the entropy of supercooled liquid decreases rapidly on cooling towards the kinetic glass transition temperature, Tg, and extrapolates to the entropy of the crystal not far below Tk (≅ Kauzman temperature ). Jakov I. Frenkel (1894-1952), famous for his fundamental book „Kinetic Theory of Liquids”, factually introduced the concept of disorder. Sir Nevill F. Mott (1905-1996) elucidated electronic structure of magnetically disordered amorphous semiconductors and resolved transition between metallic and nonmetallic states (Mott transition). Jan Tauc (1922-2010), US citizen (formerly from the Czech Republic), instigated the concept of amorphous semiconductors recognizing that the band gap does exists in spite of the absence of atomic long-range order and is not empty containing thus an appreciable amount of localized states but having its boundaries no more sharp-edged. Miloš B. Volf (1915-1983) a forgotten Czech engineer who published several book with inventory account of important glass characteristics. Raw below: Norbert J. Kreidl (1904-1994) unforgettable personality, Austrian-Czech-US guru of glass science, who promoted study on glass structure, optical properties and technology greatly assisting international cooperativeness. Charles A. Angell (1933-) studied the structure of liquids and their thermodynamic and transport characteristics, revealed understanding of undercooling and fragility. Next fife person examples show particular trends in the research and application of glasses provided by e.g.: Larry L. Hench (1937-) University of Florida, introduced bioactivity of glass-ceramics materials and their dental implant potential and contributed involved chemistry, biology and crystallization footing; Hywel A. Davies (1941-) University of Sheffield, improved science and technology of solidification at ultra high cooling rates ensuing amorphous, nanostructured and magnetic alloys; David L. Pye (1937-) Alfred University, fundamental work in nucleation and crystallization, nuclear waste vitrification, and general support of international cooperation among glass scientists, recently instigated a new international journal on Applied Glass Science; Hiroshi Suga (1930-), Osaka University, provided basis of glassines and glass-formation, analyzed molecular glasses and did heat capacity measurements during glass transformation at low temperatures, Bernhard Wunderlich (1932-) University of Tennessee at Knoxville and Rensselaer Polytechnic Institute, granted access to macromolecules crystallinity and various states of glassines existing between liquid and solid. (A wide sphere of crystallization purposefully omitted, see the preface). Separate photos shown in the below introduction: David Turnbull (1915-2007) brilliant material scientists performing research into nucleation of structural transformations demonstrating that such complex processes could be quantitatively understood. Arnošt Hrubý (1919-) was an accomplished technologist who synthesized and analyzed thousands of chalcogenide compounds and glasses and investigated their thermal behaviors mainly by means of DTA.
Fig. 13. The 2001 composition of TC7 committee (of ICG) working in the configuration (from right) G. Völksch (Germany), V.M. Fokin (Russia), M. Davis (USA), R. Müller (Germany), late P. James (UK), kneeing E. Zanotto (present chairman,, Brazil), late M. Weinberg (USA), W. Hölland (past chairman , Liechtenstein), T. Kokubo (Japon), I. Szabo (Hungary), I. Donald (UK), L. Pinckney (USA), W. Panhorst (former chairman, Germany), J. Šesták (Czech Republic). It is worth noting that the theoretical progress is still ongoing particularly pointing out, for example, novel frontiery means of a geometrical modelling based on an abnormal pen- tagonal assemblage (notoriously known to be incapable to fill any area completely with its irregular 5-fold symmetry) but yet providing possibility to arrange a surface and/or space tilling in an asymmetrical but repeating manner [7]. These yet avant-garde constructions have already found applicability in portraying the cluster structure of liquid water [9] and may become useful in describing the randomness state of non-crystallites and glasses [2], see the first paragraph. Worth of a further accentuating is also the long-lasting promotion on glass carried out by the International Commission on Glass (ICG), outstandingly achieved by its Committee on glass nucleation and crystallization (abbreviated ‘CT 7’, see the next Fig. 13.) toward the progress of glass science and technology of mostly inorganic systems based on oxide.
Since the thirties the conferences on oxide glasses started and their regular series were later followed by conferences on non-conventional glasses (chalcogenides in 50s, metals in 70s and halides in 80s). An extensive search for novel glass with properties not previously known or studied was launched within which prevailed chemical-like studies trying to find (for a given method of a technologically convenient cooling) a relevant composition with advantageous properties. It was located in contrary to an avant-garde physical approach enduring to find a fast enough method of quenching for every-individual (given) material [among others: 109,126,132,150,154]. It resulted in the introduction of novel families of unconventional glasses; first varieties, worth mentioning, are the non-oxide glasses of chalcogenides [109], metals [110-113,119] or fluorides [120], which exhibited many general features shared with oxide glasses. Serial interdisciplinary conferences were started in the fifties and most wide-spreading came later with the challenging development of xerography, electro-photography, lithography, etc.. Afterward, there appeared quite unexpected inorganic systems of which halide, oxynitrade and metallic glasses are most notable. Since the sixties those motivation unlocked the regular meetings scattered to multiple specializations and applications. Halide glasses, for example, have a potential for exploitation as ultra low-loss optical fibers (operating in the mid IR and nonlinear optics), while "metglasses" and nowadays a more sophisticated nano-crystalline "finemetals" have already found their place in various magnetic materials (exhibiting parallel spins in crystalline ‘ferro-magnetic’ and disparate spins in glassy ‘spero-magnetic’ states [119]). Worth noting is also the underpinning of glassy-like carbon in the 1960s, which is achieved by solid carbonization of thermosetting resins (once used for inert implants) that appearing in the same year as the first metallic glass by splat cooling of Au-Si alloys [110-113]. There, however, was an almost parallel progress of the particularized description of vitrification and crystallization in different material’s branches all based on the divergent but consequently unified theories of nucleation and crystal growth, which was also true for another, previously somehow separate, makeup of organic and polymeric glasses of various macromolecular systems?
Nowadays the formerly variant theories have marched to amalgamation [e.g., 122- 138,147-149,158-161]. Among many others let us just mention that well before the development of any generalized nucleation theory for condensed systems, Tammann [96] already called earlier attention to a tendency revealing that the higher the melt viscosity at the melting temperature, the lower is its crystallizability. Qualitatively, this tendency can be explained by an increased inhibition of motion or molecular rearrangement of the basic units of any melt with increasing viscosity. Later, for chalcogenide systems, Hrubý [163] defined a phenomenological glass-formation tendency more rigorously as the ratio between the differences of temperatures of the crystallization Tc, melting Tm and glass transition Tg ,( i.e. (Tg-Tc)/(Tm-Tg) [72, 163,164] – cf. previous Fig. 8). Regarding the reduced glass transition temperature, Tgr, [153] Zanotto concluded [165] that glasses having Tgr (= Tg/Tm) higher than ~0.59 display only surface (mostly heterogeneous) crystallization, while glasses showing volume (homogeneous) nucleation posses Tgr < 0.59 [165] (as supported by experimental nucleation data for abundant silicate glasses). He also assisted creation of a rather consistent theoretical basis summarised in ref. [166], which alternatives are also the subjects of the book.
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