Ferroelectricity Was Devel- Oped in Analogy with Ferromagnetism, In- Cluding the Very Name Ferroelectricity

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in the field of ferroelectricity was devel- oped in analogy with ferromagnetism, in- cluding the very name ferroelectricity. Most useful ferroelectrics possess high Ferroelectricity: dielectric constants and follow Curie– Weiss behavior near the Curie point where the material transforms from the ferroelectric state to a high-temperature The Foundation of paraelectric state. All ferroelectrics are also piezoelectric, pyroelectric, and electro- optic. It is these five phenomena, dielectric a Field from Form hysteresis, electric permittivity, piezoelectricity, pyroelectricity, and electro-optic behavior, that make ferroelectric materials useful. High permittivity is used in capac- to Function itors, piezoelectricity in electromechanical transducers, pyroelectricity in infrared imaging systems, electro-optics in photonic R.E. Newnham and L. Eric Cross communication, and dielectric hysteresis in nonvolatile memories. Doped ferroelectrics are also excellent semiconductors Abstract that are widely used as PTC (positive temperature coefficient) thermistors. This article highlights the major role Arthur von Hippel and the Laboratory for Insulation Research at the Massachusetts Institute of Technology played in the early Discovery of Ferroelectricity in development of the field of ferroelectricity in mixed oxides with the perovskite structure BaTiO3 and, in particular, in the identification of ferroelectricity in barium titanate following its One of the major turning points in ferro- discovery in industrial laboratories in the United States during World War II. Very early electricity came during the 1940s with optical and x-ray studies highlighted the characteristics of the ferroelectric domain the discovery of a number of simple structures in both ceramic and single-crystal BaTiO3, the elimination of domains at the mixed oxides that crystallize with the pe- Curie temperature TC, and the salient characteristics of the two low-temperature phase rovskite structure. Prior to the perovskite transitions. Perhaps the culmination of this work was the detailed studies of lamella 90° era, ferroelectricity was mostly a scientific domains by Peter Forsbergh and the gorgeous patterns these could generate. This curiosity unique to two rather friable article also traces the manner in which the early studies contributed to whole industries water-soluble crystals, Rochelle salt based on perovskite ferroelectrics. The ceramic capacitor industry is now fabricating (sodium potassium tartrate tetrahydrate) sophisticated, cofired multilayer capacitors (MLCs) with up to a thousand 1-µm-thick and KDP (potassium dihydrogen phos- dielectric layers interleaved with base metal electrodes, addressing a market for some phate), and at that time was thought to be 1013 capacitors per year. an order–disorder phenomenon associ- Manufacturers of large piezoelectric transducers depend almost exclusively on ated with the hydrogen bonds. perovskite-structure oxide ceramics. Navy sonar systems are major customers, but Arthur von Hippel’s discovery of ferro- spinoff has occurred into a wide range of commercial and medical ultrasound systems. electricity in barium titanate ceramics The capability of current materials has improved more than tenfold over the original changed all that. The first indications of unusual dielectric behavior in mixed BaTiO3 ceramics as a result of the effective application of molecular engineering, a strong testament to the insight of the founder of this area of study. oxide ceramics came from work carried out in industrial laboratories. Probably the first dielectric measurements were by Arthur von Hippel, barium titanate, BaTiO , capacitors, ferroelectric, Keywords: 3 Thurnauer and Deaderick at American interdisciplinary, Massachusetts Institute of Technology, materials research, MIT, Lava Co. (later acquired by 3M) in 1941.1,2 perovskite, transducers. Their high permittivity value of 1100 was confirmed by measurements at Erie Resis- tor Corp. A more detailed study by Wainer Ferroelectricity and Salomon3 at National Lead Co. (now Ferroelectricity is perhaps even today al- Ferroelectricity is defined as a physical NL Industries) was the first to show the most as much an art as it is a science. Dur- phenomenon in which a spontaneous dielectric constant peak near 125°C and ing the last 60 years, ceramists have electric dipole moment can be reoriented also to demonstrate that the peak could be identified hundreds of new ferroelectric from one crystallographic direction to an- shifted to room temperature by solid solu- oxides with many important applications. other by an applied electric field. The re- tion with strontium titanate, making the Ferroelectricity involves a complex inter- orientation process involves two or more ceramic more useful as a capacitor. Al- play of electrical, mechanical, and thermal domain states within the crystal (or within though hampered by wartime secrecy, the effects near a displacive phase transforma- individual grains in a ceramic). The key information reached Europe by 1944. tion. On cooling through the phase transi- experiment is the existence of a hysteresis Wainer’s work was submitted for publication, the symmetry is lowered and loop between polarization and electric tion in January 1943, but was not released beautiful polar domain patterns are ob- field, analogous to the ferromagnetic hys- for publication until 1946.4 At about this served in the low-temperature ferroelec- teresis loop between magnetization and time, first publication of the discovery was tric state. magnetic field. Much of the terminology also appearing in Britain.5 Independent

MRS BULLETIN • VOLUME 30 • NOVEMBER 2005 845 Ferroelectricity: The Foundation of a Field from Form to Function

discovery was also reported from Russia ica Inc.). The idea of piezoelectricity in a in 19466 and from Japan in 1946.7 randomly oriented polycrystal was at that Structurally, barium titanate has long time so incomprehensible that the great been recognized as a member of the cubic Hans Jaffe, father of the later lead zir- perovskite family.8 The most important conate titanate family (today the domi- ferroelectric materials crystallize in the pe- nant piezoceramic), testified at the legal rovskite structure. This structure may be hearing which established priority of the described as a simple cubic unit cell with Gray patent that it was only after discus- a large cation (A) on the corners, a smaller sion with von Hippel that he understood cation (B) in the body center, and oxygens how ferroelectric BaTiO3 ceramic could be (O) in the centers of the faces. The struc- piezoelectric without a continuous biasing ture is a network of corner-linked oxygen electric field. octahedra, with the smaller cation filling Matthias and von Hippel17 were the the octahedral holes and the large cation first to study the effects of electric fields on filling the dodecahedral positions at the the ferroelectric domain patterns, as corners of the unit cell (Figure 1). shown in Figure 3. Optical microscope im- The presence of a tetragonal distortion ages in polarized light revealed the sym- at room temperature was first reported by Figure 2. Dielectric constant measure- metry changes in the BaTiO3 single crystal 9 10 Rooksby and Megaw. As with many im- ments on ceramic BaTiO3 carried out by with decreasing temperature as the spon- portant discoveries, the huge dielectric von Hippel and co-workers in 1945.12 taneous electric polarization changed Dielectric hysteresis established the permittivity of BaTiO3 ceramic stimulated from one of the fourfold 001 axes to one interest throughout the world. The key existence of ferroelectricity in the of the twofold 110 axes to one of the tetragonal, orthorhombic, and measurements which established its ferro- threefold 111 axes. The corresponding rhombohedral states of BaTiO3. The electric character, however, were those of three peaks in the permittivity point group symmetries are m3m to 4mm Arthur von Hippel and his group. correspond to the transition to mm2 to 3m. It is the presence of the three During World War II, von Hippel and temperatures. The dependence of the ferroelectric phase transformations that his co-workers in the Laboratory for Insu- dielectric constant on applied field is make BaTiO3 such a useful capacitor di- lation Research at the Massachusetts Insti- associated with domain wall motion. electric over a wide temperature range. tute of Technology were engaged in the Another very important observation by study of ceramic dielectrics for use in The MIT results were rapidly confirmed Matthias and von Hippel was the much military microwave systems. Beginning by Wul and Goldman in Russia.13 Von higher dielectric constant perpendicular with the empirical data of Wainer and Hippel published this great discovery to the spontaneous polarization, just the Salomon,3 a systematic investigation of after the war14 and then went on to study opposite to that in Rochelle salt and KDP BaTiO3 ceramics was carried out at the the piezoelectric properties of electrically and critical for the high piezoelectric re- laboratory. Dielectric hysteresis measure- poled BaTiO3. sponse in the ceramic. ments immediately established the exis- The first publication on poled barium ti- Peter Forsbergh, another of von Hip- tence of ferroelectricity below 120°C, as well tanate ceramics was by Roberts,15 a gradu- pel’s graduate students, measured the as the existence of two additional phase ate student working with von Hippel at birefringence in BaTiO3 crystals and made changes below room temperature.11,12 Di- MIT, and the first reduction to practice extensive observations of the domain 18 electric data collected at five different field was a piezoelectric BaTiO3 phonograph structure. Under changes in external strengths are shown in Figure 2. pickup by Gray16 of Erie Technological conditions, domain patterns often change Products (now Murata Erie North Amer- principally by the appearance and growth

Figure 3. A multidomain crystal of BaTiO3 (a) before and (b) after the temporary application Figure 1. The perovskite structure, as of a dc electric poling field. Variations in the pattern under ac fields were also observed 17 typified by BaTiO3 above its Curie point. using stroboscopic illumination. (No scale given in original figure.)

846 MRS BULLETIN • VOLUME 30 • NOVEMBER 2005 Ferroelectricity: The Foundation of a Field from Form to Function

of thin laminar wedges, the length of new ferroelectric single crystals devel- which changes smoothly with the applied “No longer shackled to oped for the Office of Naval Research are field or mechanical stress. These 90° walls providing tremendously improved per- are both ferroelectric and ferroelastic, presently available materials, formance in sensing and actuation for whereas antiparallel domains separated we are free to dream and both Navy and medical ultrasound sys- by 180° walls are only ferroelectric and the tems and are moving effectively into com- co-joining domains require the presence of find answers to mercial production. a transverse field to become optically dis- unprecedented Many other areas of application draw tinguishable. Thin, wedge-shaped do- challenges.” on the unusual properties and property mains are illustrated in Figure 4. combinations in ferroelectric oxide systems. Comprehensive accounts describing the —Arthur von Hippel Currently, there is substantial activity in properties of barium titanate ceramics and pyroelectric properties for long-wavelength single crystals were published by von infrared thermal imaging, in electro-optics Hippel19 and by Forsbergh.20 ium titanate still dominates the ceramic for modulation and switching in guided The atomistic origin of ferroelectricity capacitor market, which has expanded to wave structures, and in nonlinear optics involves the movements of ions in the pe- some 1013 units per year. Developing an for quasi-phase-matched optical harmonic rovskite unit cell of BaTiO3. Working in understanding of the dielectric, the control generation. Thin ferroelectric films have von Hippel’s laboratory, Howard Evans of its ferroelectric characteristics by the mi- reawakened interest in ferroelectric random- was probably the first to determine the crostructure, and its defect structure have access memories, and smart cards that in- motions by x-ray diffraction.21 The crystal led to the modern multilayer ceramic ca- corporate limited memory capability are structure of the high-temperature hexago- pacitor (MLC), a technological tour-de- already in production in Asia. nal phase was also determined in the Lab- force incorporating up to a thousand oratory for Insulation Research,22 and von 1-µm-thick layers cofired with interleav- Conclusion Hippel’s group was among the first to rec- ing base metal electrodes. Units covering As a professor of electrical engineering, ognize the importance of infrared lattice the capacitance range from picofarads to Arthur von Hippel was heavily involved modes and their relation to ferroelectric- millifarads with temperature-controlled in the optimization of capacitor and trans- ity.23 The “soft mode” model for ferroelec- permittivity, very low electrical series re- ducer compositions, but he also took a tricity followed a few years later.24 sistance (ESR), and incredible reliability broad view of the role of ferroelectric stud- Much of the early history of BaTiO3 is are now widely available. ies in the development of materials sci- described in von Hippel’s books Dielectric The second major area of application ence: the understanding of solid-state Materials and Applications25 and Dielectrics in ceramic piezoelectric transducers has phase transitions, the evolution of soft and Waves.26 More recent updates are in contributed in no small measure to the lattice mode behavior, and the emergence Molecular Science and Molecular Engineer- current U.S. dominance of the marine en- of new evidence for substantial order– ing27 and The Molecular Design of Materials vironment. Piezoelectric ultrasonic trans- disorder in barium titanate. Developing a and Devices.28 ducers provide the eyes and ears for the detailed understanding of the domain U.S. submarine fleet and early warning of structure so elegantly displayed in the Ferroelectric Products any intruders in the world’s shipping early MIT optical studies is a continuing It is interesting to note the massive de- lanes. Over time, a huge range of spinoffs problem, with new evidence that the 90 velopments in electronic ceramics that has occurred into commercial and medical wall itself may be an origin for enhanced have stemmed from the early work on fer- products so that ultrasonic tomography dielectric permittivity. roelectricity in barium titanate. Now, has become a preferred technique in non- Many of these recent ideas and applica- more than 60 years after its discovery, bar- destructive medical diagnostics. Currently, tions were anticipated by von Hippel. In 1962, he wrote:

We begin to design materials with pre- scribed properties, to understand the molecular causes of their failings, to build into them safeguards against such failure, and to arrive at true yardsticks of ultimate performance. No longer shackled to presently available materials, we are free to dream and find answers to unprecedented challenges. It is this revolutionary situation which makes scientists and engineers true allies in a great adventure of the human mind.29

Arthur von Hippel was a true molecular engineer. Acknowledgments Arthur von Hippel was a pioneer in the Figure 4. A photomicrograph of 90° wedge domains in a single crystal of barium titanate history of ferroelectric materials and de- viewed in polarized light.18 (No scale given in original figure.) vices. Robert Newnham has fond memo-

MRS BULLETIN • VOLUME 30 • NOVEMBER 2005 847 Ferroelectricity: The Foundation of a Field from Form to Function

ries of many happy days at MIT. The Lab- Manufacturing Co., Electrical Report No. 8 14. A. von Hippel, R.G. Breckenridge, F.G. Ches- oratory for Insulation Research was the (September 17, 1942). ley, and L. Tisza, Ind. Eng. Chem. 38 (1946) p. 1097. prototype of a materials science center, 4. E. Wainer and A.N. Salomon, Titanium Alloy 15. S. Roberts, Phys. Rev. 71 (1947) p. 890. with engineers, chemists, physicists, and Manufacturing Co., Electrical Report No. 9 16. R.B. Gray, U.S. Patent No. 2,486,560 (January 9, 1943). (November 1, 1949). ceramists working together on problems 17. B.T. Matthias and A. von Hippel, Phys. Rev. 73 of mutual interest. Few of the modern in- 5. P.R. Courtney and K.G. Brand, Nature 157 (1946) p. 297. (1948) p. 1378. terdisciplinary laboratories have achieved 18. P.W. Forsbergh Jr., Phys. Rev. 76 (1949) p. 1187. 6. B.M. Wul and J.M. Goldman, Dolk. Akad. a comparable degree of scientific integra- 19. A. von Hippel, Rev. Mod. Phys. 22 (1950) p. 221. tion. Professor von Hippel supplied the Navk. S.S.S.R. 46 (1945) p. 154. 20. P.W. Forsbergh Jr., in Handbuch der Physik, enthusiasm and breadth of understanding 7. S. Miyake and R. Ueda, J. Phys. Soc Japan 1 Vol. XVII, edited by S. Flügge (Springer, Berlin, which made it work. His students will re- (1946) p. 32. 1956) p. 263. member the round-table discussions on 8. V.M. Goldschmidt, Skrifter, Norske Videnskap- 21. H.T. Evans, MIT Tech. Rep 58 (1953). molecular engineering in which we met sakademi, Matematisk-Naturvidenskapelig Klasse 2 22. R.D. Burbank and H.T. Evans, Acta. Cryst. 1 (publications of the Norwegian Academy of (1948) p. 330. with experts from many fields and to- Sciences, Oslo, Mathematics and Natural 23. J.T. Last, Phys. Rev. 105 (1957) p. 1740. gether outlined a vision of the future. Sciences) (1926) p. 8. 24. W. Cochran, Phys. Rev. Lett. 3 (1959) Many scientists converse fluently on 9. H.P. Rooksby, Nature 155 (1945) p. 484. p. 412. their own specialty—but few show an 10. H.D. Megaw, Nature 155 (1945) p. 485. 25. A.R. von Hippel, ed., Dielectric Materials and equal interest in the work of others. It 11. A. von Hippel, R.G. Breckenridge, A.P. de Applications (MIT Press, Cambridge, Mass., 1954). 26. A.R. von Hippel, Dielectrics and Waves (John takes a big mind and a big heart. Bretteville Jr., J.M. Brownlow, F.G. Chesley, Wiley and Sons, New York, 1954). G. Oster, L. Tisza, and W.B. Westphal, NRDC 27. A.R. von Hippel, Molecular Science and Molecu- References Report No. 300 (August 1944). lar Engineering (MIT Press and John Wiley & Sons, 1. H. Thurnauer, The Rochester Engineer 21 12. A. von Hippel, R.G. Breckenridge, A.P. New York, 1959). (1942) p. 74. de Bretteville Jr., and J.M. Brownlow, NRDC 28. A.R. von Hippel, ed., The Molecular Designing 2. H. Thurnauer and J. Deaderick, U.S. Patent Report No. 540 (October 1945). of Materials and Devices (MIT Press, Cambridge, 2,479,588 (October 21, 1941). 13. B.M. Wul and J.M. Goldman, Dolk. Akad. Mass., 1965). 3. E. Wainer and A.N. Salomon, Titanium Alloy Navk. S.S.S.R. 49 1945 (p. 179). 29. A.R. von Hippel, Science 138 (1962) p. 91. ■

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848 www.mrs.org/publications/bulletin MRS BULLETIN • VOLUME 30 • NOVEMBER 2005