Journal of Research of the National Bureau of Standards Vol. 58, No.2, February 1957 Research Paper 2736 Classification of Perovskite and Other AB03-Type Compounds l Robert S. Roth
A classification of A +2 B+'03 compounds has been made on t he basis of ionic radii of the constituent ions. A graph of t his type for t he perovskite compounds can be ~ivid e d in~o orthorhombic, pseudocubic, and cubic fi elds, wit h an area of ferroelectn c and antlferroelectnc compounds superimposed on the cuhic fi eld. The structures of sohd solutIOns . b e~ ween vari ous perovski te compounds cannot b e completely c<;)JT e l ~ted on the basIs oxides of trivalent ions has shown that all compounds of this typ~ having the pcrovs kite structw·e can be divided into two groups, with rhombohedral a nd orthorhombic sy mmetry. No ideal cubic perovski tes of the A+3B+3 0 3 compounds have been found. 1. Introduction then :fir ed in a conventional platinum-wound quench furnace, or refired in the original calcining furnace. A partial smvey of the reactions occmring in The quenching technique was used, whenever appli binary oxide mixtures of the types AO :B02 and cable, because it has been observed that sharper A20 a:B20 a has been conducted as part of a program X-ray patterns arc often obtained by very fast coo~ of fundamental research on ceramic dielectrics. In ing of the specimen. It is known that phase tranSl addition, binary, ternary, and quaternary reactions tions in the perovskite structures arc usually com between end members of the AO·B02 compounds pletely reversible and cannot be frozen in by qu ench h ave also been studied. Combinations of oxides ing. Thus the perovskite structm·es discussed are giving the simple formula type AB03 were selected the room-temperatme stable forms, although other because of current interest in ferroelectric ceramics. m aterials t reated in this manner would generally Previous publication have covered the data com contain the high-temperature forms, if any. The piled on many of these systcms, such as CaO-Ti02 fmal firing temperature ranged from 1,250° to 1,550° [1] 2 PbTiOa-PbZrOa [2], B eO with Zr02, Ti02, and C and was m aintained constant for a given length of Ce'0 2 [:3], MgO-CaO-Sn02 and Ti02 [4], :MgO-Zr02- time. Equilibrium conditions were usually reached Ti02 and CaO-Zr02-Ti02 [5], alkaline earth oxides in Ie s than 3 hr. Equilibrium was believed to have with Ge02 [6], PbTiOa-PbZrOa-PbO:Sn0 2 and been reached when X-ray patterns of the specimen PbTiOa-PbHfOa [7], alkaline ear th oxides with U0 2 showed only a sin gle phase or when the pattern of a [8], and divalent ions with Zr02 [9] . This paper is multiple-phase specimen showed no change with suc designed primarily to present new data and a new cessive heat treatment. The fired disks wer e exam method of represen tation of these structme types ined by X-ray diffraction, using a G e i ger- count~r and to coordinate the information already published. diffractometer employing nickel-filtered copper rach ation. The X-ray data reported can be considered 2 . Sample Preparations and Test Methods to be accurate to ± O.OOl A when three decimal places are recorded, and to ± 0.01 AOwh en only two In general the materials used were the highest decimal places are recorded. The results reported avail able purity of the component oxides, varying in this work are gener ally the data obtained at room from about 98.5- to 99.9-percent pmity. The start temperature from specimens treated in a m anner to ing materials, in suffi cien t quantities to give either form a matured ceramic dielectric bodv. a 10.0-g sample or a 1.0-g sample, depending on the In the case of specimens containing an ion that was avail ability of the raw materials, were weighed to apt to oxidize, a neutral atmosphere was m aintained the nearest milligram, mixed together, and formed in the furnace. These ions were VH , UH, and Ce+3 ; into I-in. or X-in.-diameter disks at a pressm e of 2 the neutral atmosphere used was either helium or 5,000 Ib/in. The pressed disks were fired for 4 hr argon. The fm·naces used were similar to those at 1,100° C on platinum foil in an air atmosphere, previously mentioned, bu t modi:lied to maintain a u ing an electrically heated fm·nace wound with neutral atmosphere. 80-percent-P t 20-percent-Rh wire. These disks were then ground and remixed, and 3. The Perovskite Structure Type ne\v disks for study of the solid-state reactions, about X in. high, were formed at 15, 000 Ib/in.2 in either a The structures of the perovskite-type compounds }~ -i n . or X-in.-diameter mold. The specimens were have been studied by many workers. The exact
I This work has been sponsored as part of a program for Improvement of nature of the structmes involved is still in doubt Piezoelectric Cerami cs by the omce of Orcinance Research of tbe Department in many cases. Conflicting reports in the literature of the Army. o Figures in brackets indicate the literatm e references at the end of this paper. make difficult the job of assigning the correct struc- 75 trn'e, or even symmetry, to any particular perovskite classify more than one valence group of structure type compound. The simplest case of the perov types on anyone diagram. In other words, separate skite structure is that of a simple cubic cell with diagrams are needed for the compounds of the type one ABX3 formula unit per unit cell. In this case A +2B H 0 3, A +3B+30 3, A +lB H 0 3, etc. It seems quite the A ions are at the corners of the unit cell with probable that perovskite-type structures with F-l, the B ion at the center and the negative ions occupy or some other such ion, in place of 0 - 2 would also ing the face-centered positons. The space group require separate diagrams for a reasonable classifi is 0~-Pm3m and has the notation G-5 of the Struk cation. These diagrams still do not lend themselves turbericht [10]. As will be seen in the following to absolute classifications, as some compounds simply discussion of actual perovskite-type compounds, do not seem to fit, It has been suggested by Roberts very few binary oxides have this simple cubic [20, 21] that the polarization of the constituent ions structure at room temperature, although many of may play an important role in determining the sym them assume this structure at elevated temperatures. metry formed by a given pervoskite structure. A Many modifications of this simple structure have classic example of polarization determining crystal been proposed to account for the X -ray patterns type over and above ionic radii may be found in observed for different perovskite-type compounds. the phenacite-olivine structure types where the Zn+2 Among these modifications a simple doubling of the ion is found to be out of place in a radii classification unit cell has been used [11]. A monoclinic modi [22] . It is this notion of polarization that is used fication suggested by N aI'ay-Szabo [12] for the type in the present paper in an attempt to make a better compound perovskite (CaTi03) has been shown to classification and diagrammatic representation of the be actually orthorhombic by M egaw [1 3]. The perovskite structures. latest modification of this structure was made by Bailey [14] and quoted by Megaw [15], using an 4 . Results and Discussion orthorhombic modification with the b parameter approximately twice the pseudocubic cell edge and 4.1. Mixed O xides of Divalent and Tetra valent Ions with the a and c parameters approximately..j2 times a. General the pseudo cubic cell. It will be seen that the X-ray For the purpose of convenience most of the data patterns of a great many compounds can be indexed accumulated has been put in the form of tables, on the basis of this type of distortion of the perov although some of the more controversial composi skite structure. The compounds CaTi03, CaZr03, tions are also discussed in the following sections. and CaSnOa have been indexed previously on this It is known that CdTi03 crystallizes in both the basis and the d-spacings and (hkl) values published perovskite- and ilmenite-type structures [23]. As the [4, 5]. However, it should be noted that a dis perovskite structure requires a relatively large A ion crepancy exists between the reported space group and the ilmenite structure a relatively small A ion, for CaTi03, Pcmn [15] , and the published indexing it can be seen that all AB03 compounds with Ti+4 of these patterns. Although CaSn03 contains no and an A+2 ion larger than Cd+2 should have the reflections that are not allowed by this space group, perovskite structure, whereas those with A +2 ions both CaTi03 itself and also CaZr03 show diffraction smaller than Cd+2 should have the ilmenite structure. peaks, which can only be indexed on the basis of In all such cases where compound formation takes forbidden reflections. These peaks are especially place, this is found to be true. The values used in prominent for CaZr03 and indicate that perhaps a this study, for the radii of the metallic ions under different space group is r equired for the CaTi03- consideration, are listed in table 1. The composition, type orthorhombic modifications of the perovskite structure type, and symmetry, where known, for the structure. mixed oxides of divalent and tetravalent ions are In addition to those types of distortions that listed in table 2. The parameters of the unit cell require a multiplication of the pseudo cubic cell, are given for all those materials studied. Informa there are several other distortions in perovskite tion obtained in the present study is shown in bold type structures that require only a slight modifi faced type, and the author's comments on previous cation of one or two parameters, resulting in tetrag work are given in italics. In general, references to onal, orthorhombic, and rhombohedral symmetries. other works are made only when the particular It is these latter modifications that are of most compound or mixture has not been studied in the interest in the study of ferroelectric forms of the present work or when results of other investigators perovskite structure, especially in the two groups disagree with the results found in the present investi of mixed oxides under discussion, A +2B+4 0 3 and gation. A +3B +30 3. Other ferroelectric perovskite com b. Ce0 2:Ti0 2 pounds, like Na+1NbH 0 3, have been suggested to A specimen of Ce02:TiOz heated in air has yielded have multiple-type unit cells [16] even larger than in an X-ray pattern of Ce02 plus a perovskite-type the CaTi03 type. phase. It seems highly unlikely that either Ce02 or A classification of the perovskite-type structures Ti02 would reduce, in air, to the poin t of forming an on the basis of the radii of the constituent metallic A+Z B +10 3 structure. It seems more likely that an ions has been attempted by several workers [17, A+3B +30 3 structure is the phase found in the X-ray 18, 16, 19]. It has been pointed out by Keith and pattern (see discussion of classification of mixed Roy [19] that it is not-, advisable to attempt to oxides of trivalent ions). 76 TABLE 1. Radii and polarizability, where c. SrZrOs used, of metallic ions pertinent to this study The compound SrZrOa has generally been regarded Ion as a cubic perovskite [24, 16, 25]; although N aray I R adius • [ i z~~)~:f~~~ "I Ion IRadi us a Szabo [26] has referred it, like CaTiOa, to the mono ----'-----'---1 clinic system. The X-ray pattern of this compound shows diffraction lines other than those found in ---,---- -,------simple cubic structures, definitely indicating some A A ' A BaH 1_ 34 70_6 1\1n+2 0_ RO type of distortion of the unit cell. In addition, the PbH 1. 20 91. 8 "Fr+2 . 74 "cubic" peaks in the SrZrOa pattern have a ten Eu +~ b 1. 16 Zn +2 .74 81'+2 1. 12 49_1 00+2 .72 dency to split into two or more peaks (not Kal and Ca+Z 0_99 34_9 N i+2 _09 Od +: _97 ' 56_9 Mg+' .66 K(2)' The Ka2 doublets cannot be clearly dis Be+:? .35 tinguished in the SrZrOa X-ray pattern, although ------'---1 thcy are quite clearly shown in patterns of the true ~ L"'l'i valont ions cubic perovskite specimens like BaZrOa or SrTiOa. The split in the pseudocubic peaks resembles those AI +' 0.51 Bi+3 0.96 Oa+3 .62 Gd +' _97 found in PbZrOa for a pseudo tetragonal distortion Or+3 .63 Sm+3 1.00 Fe+3 _64 N d+3 1. 04 with cia 1, although there are a few possible di s Sc+' 0.80 00+' 1.07 crepancies in intensity valu es. On the other hand, In +3 c _ 82 La+3 1. 14 y +3 _92 the distortion may be of the type found in CaTiOa. As observed in at least six different specimens, the 'l'ctf'avnicnt ions SrZrOa " pseudocubic" peak at a value of h2+ fc2 + l2= 6 is split into a close doublet with the low-angle side Ot< 0,16 0_71 SiH ,42 . 7~ of the doublet lower in intensity than the high-angle GcH .53 .79 side. The same peak in PbZrOa has the intensities 1\I[n H _60 _94 V" ,6:1 .97 of the doublet rever cd. Although CaZrOa and rri H _68 1.02 CaTiOa h ave foul' peaks very close together in this region, the two on the low-angle side arc of lower a Unless othel'wise indicated, rad ii of the ions arc talrcn from A hrells [301. intensity than the two on the high-angle side. Such b'rho radius of Eu+2 h 1S beon adapted from the discrepancies, noted above, make it impossible to value given by (, rcen [3ll and corrected for rad ius of 0 -'=1.40A. assign SrZrOa to either the PbZrOa 01' CaTiOa struc o 'rhe radius of In+3 and 8c+3 are both givell as O.8l A by Ahrens [30], As this eannot be shown on the ture. Calculations of (hlel) values for bo th types of diagram, In+3 is used as 0,82 A and Sc+' as O.80 A. This cells, as well as theoretical intensity values for each is tbought to be justified as the compounds of Sc+' are said to ba \' e Slightly small er parameters than the structure, would have to be made before a fmal de corresponclin g eompounds of In+' [19], d Unless otherwise indicated, polarizability of the cision could be made. ions is taken from Roberts [20], • '1'bc pola)'izability of Od+' was estimated as ex ploinecl in tex t ,
TABLE 2_ Nhxed oxides oj the type A +2B +< 0 3
Toler- J leat treatment ance Composition factor for Structure type Symmetry Heferences and diseussion pcrov- 'r em perature 'rime skiLe (°0) (hI' ) stl'uctUI'Ca
Mixed oxides containing 00,
BaOO' ______Aragonite ___ . ______Orthorhombic ______Palache, Berman, ancl Fronde! [39] PbOO, ______do ______clo ______D o, SrOO, ______.______do ______do_. ______Do, OaCO' ______. do ______do ______Do _ OaOO' ______Oalcite.______RhombohedraL ______Do, OdOO, ______clo ______do ______Do _ j\[nOO' ______do ______do ______Do _ ]>e 003______.do ______do ______Do _ ZnOO'. ______do ______clo ______Do _ 0000 , ______do ______clo ______Do, j\ lgO O, ______do ______c10 ______Do,
j\1ixed oxides containing Si02 n aSiO, ______Unknown. ______. ______Present work. Specimen slLpplied by E . Levin oj NBS stuff_ PbSiO, ______Unlrnown ______Winchell [40]_ SrSiO, ______. __ _ Pseudo wollastonite . _ _ Unknown ______Present work. Specimen supplied by E. T. Carlson oj NBS staff. OaSiO, ______1.400 ______Pse udowollastonite . __ U nknown. ______Present work. Specimen supplied by 1-1. S. Yoder oj Geophysc;ul Luboratory, Carnegie Inst., Wushington, D_ C. OdSiO, ______Unknown ______. ______Silverman, Morey. and Rossini [41]_ M gSi0 3______Enstatite ______Orthorhombic ______Present work. X -ray data obtained from specimen of Bishopsville m eteorite supplied by FT, S. Yoder_ a=18_222, b=8,81O, c= 6_182 ri,
77 TABLE 2. M ixed oxides of the type A +2 B H 0 3- Continued
'r oler H eat treatment ance Composition factor for Structure type Symmetry R cferences and discussion perov 1'cmpcrature Timc skite (O C ) (hr) structurea
Mixcd oxidcs containing OeO,
1 Pseudowollastonitc _ _ _ U known ______Roth [6j' Symmetry almost Ite xagonal. BaOeO'. ______1.00 P seudowollastonitc _ _ _ Unknown ______g6~~~ _::::::::::::: 1 U nknown ______Ro(h [6. H eated at 20.000 psi. P b OeO,_ ._ ...... _ 0.95 775 ______0. 5 Present work. Probably nonequilibrium. 8rOeO, __ . _. ___ . __ . 92 1.235 ______.. __ 1 P seudo wollastonite _ _ _ U nknown ______Roth [6J. CaOeO' ______.88 1.235 _____ .... _.. __ 1 U nknown ______Roth [6]. Probably noneqltilibrium. CdOe03____ . ___ __ .87 1.100 _. __ ... _._ ... _ 1 U nknown ______Present work. Probably nonf.quilibrium. ZnO:OeO' ______.78 1.100 _.. _... _._. __ _ 0.66 No 1: 1 compound ______Present work. Speciment contains Zn2Ge04, willemite type. plus GeO,. qltartz type. MgOeO' ______• 7 ~ 1.235_ ..... _.. ____ _ Enstatite______Orthorhombic ______Roth [6]. a=18.661 . b=8.95!'. c=5.346 A. BeO:Oe0 2______.64 No information ______1: 1 compound between BeO and OeO, seem' unlikely; probably onlll Be,Oe04forms.
Mixed oxides containing ]vInO,
o 84 ' ---. ------. ---"1 pel'OVskite ______._.1 C UbiC__ ···_·······_···1 W ard. GlIsh ee. McCarrol. and Ridgely [42]. I{------Pel'ovskite __ . ______._ C ubic_._ .. _...... _... Y ak el [43]. Symmetry reported as cubic with doubled cell .
Mixed oxides conta inin~ VO,
Ba VO, __. __ . ______0. 95 1.200 (Helium)._ .. 0.5 Unknown ______• ______Present work. 8rY 0 , .... ______.88 1.200 (Helium)._ .. . 5 U nknown ______. ______. __ Present work. CaVO'. _____ . ____ _ .5 P erovskite_ .. ____ • ___ . Orthorhombic 0.,._. __ Presen1 work. a=5.326./)=7.547. c=5.352 A. .83 e :~~ -( ~~~~~~: ::::: P erovskite_. ___ . __ .... Cubic ... _...... _._. __ W ard. G ushec. McCarrol. and Ridgely [42]. M gO :VO' • ...... 72 Unknown ...... _. . _...... _... _ King and Suber [441. BeO :VO'. __ ...... Bl Unknown ...... _ ...... __ ..... _.... King and Suber [441.
Mixcd oxides containing Ti02
BaTiO,_ .. __ .. __ .. 0_Q3 1.350 __ . _. ______Perovskite_. ______. __ . Tetragonal c/a> L _._ .. Present work. a=3.989. c=4.029 A. Pb'fiO,. ____ ._.... .88 1.100_. _. ______.. _ P erovskite .... ____ .. _. Tetragonal c/a> L _.... Jalfe. Itoth. a nd Marzullo [7]. a=3.896. c= 4.136 A. E uTi03.. ______87 P erovskite_._ ... ______Cuhic. ______. ______Brous. Fankucben. and Banks [45J . SrTi03______. ___ . . 86 1.300. ___ . __ .. _._._ 1 P erovskite _.... ______Cubic __ . ______. ______Present work. a=3.904 A . CaTiO, __ . ____ ._.. . 81 > 1.700 ______. ___ ._ 1. 5 Perovskite ______Orthorhombic c ______Coughanour, Roth, Marzullo. a nd Sennett [4]. a=5.381. b=7.645. c=5.443 A. CdTiO,_. ______. } 81 Perovskite._._ ...... Orthorhombic_._. ___ .. Present work. 0=5.301. b=7.606. c=5.419 A. CdTiO, ______.____ . e :~r:~'_ ~~::::::::::: Ilmenite ____ ._._... _.. RhombohedraL ______Posn!ak and Barth [23]. Mnrri03______.75 1.250._._._ .... _... I1me nite ______RhombohedraL ______Present work. a=5.585 A, a=5It57'. Ferri03______.73 Ilmenite. ______RhombohedraL . ______Wells [24]. ZnO:TiO, ...... __ .73 1.245. ___ . _. ______No 1 :1 compound ______Present work. Specimen contains ZnzTi04, spinel type. plus TiO,. rutile. CoTiO, ______... .72 1.400_... ___ . _. ___ _ IImenite_._ ...... _. RhombohedraL _____ . Prespnt work. X·Ray pattern too diffuse to mea~ure parameters. NiTiO, .•. _.... _.. .71 1.400. __ ...... _. . . 2 I1menite ._._. ______. ___ RhombohedraL ______Present work. a=5.448 A. a=55°. M gTiO, ...... _.___ . 70 1.550. ___ . _. . ___ .. . 2 I1me nite ______RhombohedraL ______Present work. a=5.519 A, a =54°30'. B eO : TiO,_ ..... ___ .60 1.800 _. ______.. _ 0. 5 No reaction_. •.. ______._. _____ . __ ._ .. Lang.ltoth. and Fillmore [3].
Mixed oxides containing SnO,
BaSnO' _____ •... _. 0.92 1.450 .•.. ___ . _. _.. _ Perovskite______Cubic ______Present work. a=1,.114 A. 850 _. ___ . ___ ... _._. No 1:1 compound ______Jall'e. Roth. and Marzullo [7] . Specimen con· tains Pb2S n041 unknown structure plus SuO,. 650 __ .. ______24 No reaction ______Starting material PbO and SnO,. 650______0.5 U nknown ______PbO: SnO,._ ... __ _ Distorted fluorite- type structure, probably .87 PbSn04. Starting material precipitated hy· drated lead stannate. 800 __ ._ ... _...... PbSnO' ...... Unknown ______.. __ Coffen [46] . Starting material p recipitated hy· drated lead stannate. P erovskite .... ______TetragonaL._ ...... N aray ·Szabo [26]. Perovskite______Cubic ______Present work. Commercial specimen, unfired. a=1,.034 A . SrSnO'_._ ...... _. . 85 Perovskite ..... __ . _._. Cubic_. ____ ._ . ____ . __ . M egaw [13]. Coffcen [46]. Perovskite __ .... __ .. _. CaTi03 type ______. __ _ Naray·Szabo [26] . CaSnO' ...... _ .80 Perovskite ______Orthorhombic c______Coughanour, Hoth, Marzullo. a nd Sennett [4] . a=5.518. b=7.884. c=5.564 A . No compound ______Presel1t work. Specimen contained only SnO,; CdO volatilized. A compound of CdSnO, probably exists belo w this tempera· CdSnO' ______._. . 80 ture . Unknown ... _.. • _...... _.. _...... _.... _.. _._ . . Coffeen [46]. Decomposition of CdSnO, is about 1.177° C. Perovskite._._. __. __ .. Ca'l'iO, type .. _. ______:-;raray·Szabo [26] . MnO:8nO, •.• _.. _ 0.74 No reaction ______• ______Present work. MnO added as MnC03. Speci· men contained SnO,. Mn30 4. ZnSn03 (?) __ •..• _._ .72 Unknown._. ____ . ______. _.• _. __ .•....•... _. Coffeen [40] . P ublished pattern liTobably spinel plusSnO,. Present work. Specimen contained C02Sn04, 000:8nO' •.. _.... . 71 spi nel plus SnO,. t~:~~::~::::::::: --. ~ -.- ... ~:~~\:~.~~~.~~ ::::: :::::::::::::::::::::::: Coffeen L46] . Published pattern that of a mix· tUTe oj spinel and SnO,.
78 T ,IBLE 2. A!fixed oxides of the type A +2 B+403- Continuecl
'1' oler J Ica t treatment ancc Composition factor for Structure type Symmetry References and d iseussion pcrov 'r em perature 'rim e skito (O C ) (lir) structure a
Mixed oxides contain ing Sn Oz- Continu cd
NiSnO' ______. 70 U nknO\Vll ______Co ffeen [46J. Ilmenite ______Rhombohedral.. ______oo ------66 Cou ghanour, Uoth, Marzullo. and Senn ett [4]. X-ray pat/ern too diff"se to mea81 Mixed oxides containing UfO,d PbRfO, ______0.84 P erovskite______P seudotetragonai. ___ _ Shirane and Peplnsky [47J. SrH fO, ______.82 P erovskitc ______CaTiO, type ______Naray·Swbo [26]. Unlikely tliat pure RfO, was available attliis tim •. CaUfO, ______78 Pero\'skite ______CaZrO, typc ______Curtis, Doney, and Johnso n [48J. Mixed oxides contain in g ZI'OJf BaZrO' ______0.8S 1,450 ______P crosvkitc . ______Cubic ______Present work. fL=4.19f A. P erovskitc______l>scudotetragonaL ____ _ Jalre, Roth, and Marzullo [7J. a=4.156, PbZrO' ______.84 c=4.19O A. Pcrovskite______PseudotetragonaL . ___ _ Sawagllehi, Maniwa, and ITosh ino [49J. L~~~;;;;;;;;;;;;; Perovskite______PseudoietragonaL __ ._ Sawaguchi, Shiranc, and 1'akagi [50} .g 1,750 ______.. P crosvkitc______l>sc udocubic ______Present work. a=4.099 A. Starting malerial commercial 8rZ1'03, also contains excess Afonoctinic Zr02 plus unknown pl/ase. 1, 500 ___ ._------2 ]>erovs kitc ______Probably orthor hol11 - Pr cs~ nt work. a=5.792, b=8.189, c=5.818A. SrZrO, ______.81 bic. c Slartina material SrCO, pt"s Monoclinic 21'01 no e.rcess phases. ------Perovskite______Cubic ______Wells (24J, Wooel [16J, Shirane and Hosbino [25J. Perovskite ______CaTiO, type ______Naray·Szabo [26]. CaZrO' ______------.77 1.550_ ------6 P erovskitc______Orthorholllbicc ______Coughanour, Roth, M .. trzullo, and S~ nn ett [5]. a=5.587 . h=8.008, c=/i.758 A. CdO: Zr02 ______.76 ------Perovskite ______CaTiO, type (ortho- Naray-Szabo [26J. As many oftlte compo" nds rhombic). listed in til is reference do no/ actuaUy exist, CdZrO, cannot be accepted as a true com pound wilhout jurtlier experimental evidence. MnO:ZrO'_ . ______71 1,400 ______No 1 :1 compound ______Rolh [9J . Contains Cltbic Z1'O, solid solution pl"s J" I n,O•. ZnO: 7, rO , . ______.69 1,300 ______3 No reaction __ • ______Present work. CoO :Zr02______0.68 ------No reaction ______Dietzel and Tober [51l' NiO:ZrO ' ______.67 ------No rcaction ______Dietzel and Tober 151 . TiO: Zr02 ______1,420 (He liul11 ) ____ 3 No 1:1 compound ______Roth [9] . Contains cubic Z1'O , solid solulion 1Jlus rr' iO .. Specimen SlL1Jplied by F. Brown, Jet Propulsion L ab., Calif. Inst. of 'Tech. MgO:ZrO' ______.66 1,515 ______48 No 1:1 compound ______Cougha nour, Roth , Marzullo, and Se nnett [51. 1,800 ______Contains cnbic ZrO,soliri solution pt"s MgO. BeO:ZrO. ______.56 O. 5 No reaction ______Lang, Uoth, a nd ~"' illl11ore [3]. Mixed oxides containing CeO, P erovskite ______Pse udocubic ______Present work_ a=4.887 A. P erovskite ______Pseudocubic ______Present work. a=4.887 A. Possibly ortho- BaCoO' ______0. 83 rhombic. h Perovskite ______CaTiO, type ______N aray-Szabo [26]. Perovskite______Cu bic ______Hoffman [52], Wells [24J, Wood [16J. r~~ :~~~~~~~~~~~~~ No reaction ______0.5 Present work . PbO:CeO' ______.79 r~~~ :::::::::::::: Perovskite______CaTiO, type ______Naray-Szabo [26J. Compound probably does not exist. SrCeO' ______4 J>erovskite . ______Orthorhombic 0 ______Present work_. a=5.986, b=8.5~l, c=IJ.125 A . .70 Perovskite ______CaTiO, type ______'I aray-Szabo [26]. e :~~~:::::::::::::: No reaction ______No reactlon ______Present work. CaO :Ce02 ______.72 Keith and Roy [19J. r~~ :::::::::::::: Perovskite______CaTiO, type ______N aray-Szabo [26J. Compo"nd probably does not exist. No reaction ______Keiih and Hoy [19J . CdO: Ce02 ______.72 {::::::::::::::::::: Perovskite______CaTiO, typc ______Naray-Szabo [26J. Compound probably does not exist. No reaction ______Present work. No reaction ______Keith and Roy [19J . MgO:CeO. ______.62 l~ :~~~::::::: Perovskite. ______Carri03 ______N aray-Szabo [26J . Compound probably does ::: :::: not exist. Such a compound could only be an 1,800 ______antiperovskite structlLre. BeO:CcO' ______.53 0.5 No conlpound ______Lang, Roth, a nd Fillmore [3J. Possibly a solid sol"tion of BeO in CeO,. 79 TABLE 2. lIt[ixed oxides of the type A+2B+403- Continued Toler- H eat treatment ance Composition factor for Structure type Symmetry References and discussion perov- Temperature Timc ski te (OC) (hI') structure a. J\'Iixcd oxides containing lT02 i BaUO, ______0. 82 1,000 (Argon) .. ____ 0. 5 I Perovskitc . ------Pseudocubic ______Lang. Knudsen. Fillmore. and Roth [8J. a~4.387 A. Similar to BaCeO, with very small splitting of diffraction peaks. SrUO,. ______. 75 1,000 (Argon) .. ____ .5 Perovskite __~______Orthorhombic c ______Lang. Knudsen. Fillmore. and Roth [8]. a~ 6. 01. b~8.60, c~6.17 A . .5 Perovskite ______Orthorhombic c ______Lang, Knudsen, Fillmore, and Roth [8J . CaUO' ______. 71 a ~5 .78 , b~8.29 . c~5 . 97 A. t~~~ -(~~~~~~~~:::: Alberman. Blakely. and Anderson [27J .; MgO:UO'- ______. 61 1,800 (Argon) ______.5 Lang, Knudsen, Fillmore, and Roth [8J' BeO :UO, ______.52 1,800 (Argon) ______.5 ~r~:m:f~ : :::: :::::: I:~~;;~: ::::::: :::: ::::: Lang, Knudsen, Fillmore, and Roth [8 . Mixed oxides containing T hO,' BaThO,. ______1 0. 80 1::::::::::::::::::::1 :::::: 1~ ~~~~~~n~ ::::::::::::1 g ~~\'b ~'tY I;e:::::::::: 1 ~~~a~~S'zlt~ · [ ~~e ll s [24J. Wood [16J. a As the radii givpn by Ahrens [30] are closer to Pauling radii than to Goldschmidt radii. the values listed for tbe tolerance factor are, in general, slightly smaller th an thosc calculated by other authors (Keith and Roy [19J. Wood [16]), using modified Goldsc hmidt radii. b Probably only CaO can form a perovskite componnd with MnO, (Ward. Gushee. McCan'ol, and Ridgely [42]). It secms unlikely that CaMnO, has a simple cu bic structure. and it is tentatively left on the border betwee n orthorhombic and pse udocubic types. c This structure is related to cubic porovskite as follows: a~ .J2a' . b~ 2a ', c~ ..fia', d Due to the lan tbanide contraction. HfH ]]as a radius only sligh tly smaller than ZrH, and tbe compounds containing HfO, are probably m ostly isostrnctural with t.h e corresponding ZrO, compounds. e 1'ho oseudotetra£!:onal structure w ith c/a< l is actuall y orthorhombic with a= .JZ,a', b=2 .J?,a.', and c=2c'. f N o compounds of the i1m enite structure type have been found with Zr02, or, indeed, with any BH ion larger than 8n+4, g Strong anomaly of the dielectric constai,t at abollt 230° C, antiferroelectric below this tempcrature. h The exact nature of the distortion has not been determined. as the amount of distortion i, small. T hcre are definite splits ill the diffraction lines as well as excess peaks other than those fot' a Simple cubic structure. ; No work bas been reported on U O, systems with N iO , CoO. Zn 0, MuO. CdO. or FeO. ; P ublished X-ray pattern is that of a mixture of UO, solid solntion and pero vs kite compound. Pattern published for Ca,UO, is actually the perovskite-type compound. k Perovskites of the followin g ox ides with '1' hO, have heen listed by Naray-Szabo [26J: BaO. PbO. SrO, CaO, CdO. and l\ lgO. Of these reported compounds, probably only BaThO, reall y ex ists (Keith and Roy [19]). I Although no ThO, compounds were examined in the present work, it seems likely that BaThO, would have the same type of structure as is found in BaCeO, and BaUO,. d. CaU03 in a total increase in the tolerance factor. However, a replacement of 50 percent of UH by U+6 and Ca+Z The compound CaU0 3 also h as a CaTi03 structure. increases the tolerance factor to only 0.726 . Thus However, a 50:50 molecular mixture of CaO and it can be seen that the lower limit of 0.77 for the U02 does not yield an equilibrium perovskite com tolerance factor in A +2B+40 3 perovskite structures is pound. The formation of the perovskite compound not absolutely correct. in the CaO-U0 2 system takes place only in the presence of excess CaO. This phenomena h as been e. BaGeOa discussed by Alberman, et al. [27 ] and by L ang, The AB03 silicate compounds cannot form perov et al. [8]. The perovskite compounds of the type skite-type structures, as the silicon ion is much too A +2BH0 3 have been assigned a minimum tolerance small, always OCCUlTing in tetrahedral coordination. factor (t) of 0.77 by Keith and Roy [19 ]. The toler The B ion must occupy an octahedral position with ance factor for the perovskite structure, as de respect to oxygen, even in the most distorted struc scribed by Goldschmidt [17], is tUTes, in order to be classified as a perovskite. As the germanium ion is larger than the silicon ion and is known to OCCUT in octahedral coordination, a study was made of the reaction of Ge0 2and various divalent metallic oxides. The only known case of octa where hedrally coordinated GeH is one of the polymorphic t= tolerance factor for perovskite structure, forms of Ge02. Ge02 can be readily formed in the RA= ionic radius of larger cation , rutile-type structure by heating the quartz-type RB = ionic radius of smaller cation, polymorph at 700 0 C and 20 ,000 psi.3 An attempt Ro=ionic radius of oxygen (=1.40 A). was made to form a titanate-type BaGe0 3 (with the germanium ion in octahedral coordination) by heat · This tolerance factor for CaU03 is equal to 0.71 and ing the pseudowollastonite form to 700 0 C at 20,000 is the only known A +2B+40 3 perovskite-type com psi, but no change in the crystal structure was pound with t considerably less th an 0.77. Some observed. Differential thermal analyses of BaGeOa Ca+2 may actually substitute for U+4 in the B posi indicated a phase transformation at about 340 0 C.4 tion, accompanied by a partial oxidation of UH to ' A specimen of rutile-type GeO, was prepared In Ithis manner by A. Van UH, to give a structUTal formula Ca(Ut4U ~ ~1_X) Valkenburg and E . Bunting of the Bmea u stafl. • ' :-0 4 The diflerential tbermal analysis was performed by E. Newman of the Bureau Ca ~ h-X») 0 3 . Such a structural substitution results stafl. 80 +4 This phase transformation is apparently very dis Hf · J'uptive, as a pressed pcllet of BaO:Ge02 results in Si+4 ~4 M+n4 J4 T14 s~ I,Z~4 C~ y4nt4 finely divided white powder of crystalline BaGe03 1.35 '--""e-.--.-"'Ti-"T"'7rr--+-iIT-'+-'v: v, "r---r-L..LT'--'l- 80+ 2 when heated to 1,000 0 C and furnace cooled. A I PSEUDO- I 1.25 ARAGONITE I WOLLASTONI TE I I I-per cent addition of F c 0 3 to BaGe03 results in an I I I PEROVSK ITES 2 e · IS rV03 1 intact ceramic specimen with an entirely different I I I I ILMENITE I 0 .85 CALCITE i [ : NO COMPOUND a:" A classification of structure types based on the : NO CO MPOU ND l O X 1 X I" : {)onstituent ionic radii can be made for all the com .75 e II- ______X..J O X! H e pounds of the A+2B 0 3 type. However, the prop : ENSTATITE: 0 J .65 e 0 0 0 /OJ X X X erties of the solid solutions formed by a mixture of .10 .20 .30 !l0 .50 .60 .70 .80 .90 1.00 two or more compounds cannot always be correlated RADIUS OF 8+4 IONS . A by a simple radii classification. In an attempt to €xplain some of these discrepancies a third property, F I GURE 1. Classification oj the A +2B +'O,-lype com po unds polarizability of the consti tuent ions, has been used. according to the constit1tent ionic mdii. 0 . Compounds sLudied in t he present wo rk that have t he structure ShO\\Ol by It is exceedingly difficult, in the presen t state of t he areas bou nded by dashcd Jincs; ~ . compoullds not st udied in t he prescn t knowledge, to arrive at correct values for the polari WOI k that arc assumed to have the foi tr ucturc shown by the areas bounded by dashed lines; .Jco mpounds with complex or unknown structure type; X. pasi· zability of a given ion in a given structure type. The tion of cO ll1po s i Lion~ stud ied in the prf'scnt work that do not form A +2.3+4 0 3 compound s; ® , the presence of a co mpound of F e1'i0 3, and tho a. bsence of a com- values used here have bee n taken mainly from the )O unci at the composition ZnO:Ti02 are indicated t.oge ther, as ZnH and FeH l'esults published by Roberts [20 , 21 ] and arc listed Ilave approximately the same l' adillS. in table 1. However, a Roberts did not give a value for Cd+2, this ion was assigned a value that would fit the observations of other workers. Both P auling can be divided into perovskite and ilmenite structure 128 ] and Kettelaar [29] indicate that the polariza types. The boundary between the two types occurs bility of the Cd+ 2 ion in a given structure type is at a value ncar that of the Cd+2 ion and is shown as between those of Ba+2 and of Sr+2. Therefore, the horizontal for the purpose of convenience. As no values given by K ettelaar [29] for Ba+2, Cd+2, and ilmenite compounds arc known with a B+·1 ion large t' 1'+2 wwe compared, proportionately, to those given than the radius of SnH (0.71 A), the lower right by Roberts for the Ba+2 and Sr+2 ions in the perov cornel' of figure 1 is shown as an area of no compound kite s tructure, and a value of approximately 0.56 formation. In all cases the boundary lines between A3 was assigned to the Cd+3 ion. stl'uctme types arc only an approximation, as not A b asic division of structure types can be made on cIl ough information is available on mixtures of com a two-dimensional chart of AB03 compounds, wh ere pounds of two different structure types. In general, the radius of the A +2 ion is plotted as the ordinate there is only limited solid solution, if any, between and the radius of the B +~ ion as the abscissa. In compounds of diiIerent structure types, although figure 1 and table 1, all values of the radii, except there is consider able or complete solid solution be for Eu+2, arc taken from Ahrens [30] and represent tween compounds of a given type. An exception is the ionic size for sixfold coordination. As pointed found in some mixtures of compounds on adj acent out by 'iVood [16], a correction for coordination sides of the boundary between orthorhombic and would tend to move boundary lines but not to distort cubic (or tetragonal) perovskite compounds. the picture of the basic cl assification. The value for A classification of those compounds having Eu+2 has been taken from Green [31] and corrected perovskite-type structures is shown in figure 2. for a r adius of 0 - 2 of 1.40 A. In general, a division The A +2B+40 a perovskites can be divided into two {) an be made between structural types containing general types, those that are cubic or have distortions large and small A +2 ions. The boundary of this requiring an elongation of one or more parameters, division usually falls near a value of 1.0 A, and often and those that have di stortions that require some the Ca+2 (0.99 A) or Cd+2 (0.97 A) compounds occur multiplication of the busic perovskite unit cell. in both structure types. For the carbonates, all The latter type is mainly represented by the CaTiOa {)ompounds containing divalent ions larger than Ca+2 orthorhombic structure, those perovskites with have the aragonite structure, whereas those with relatively larger BH iOlls and small er A +2 ions. An ions smaller than Ca+2 have the calcite structure. intermediate group are those compounds labeled III the case of the silicates and germanates, the pseudo cubic which, although not cubic, have not pseudowollastonite structure type occurs for most of been definitely identified as to structure type. the compounds containing large divalent ions, This group separates the cubic from the orthorhombic whereas the enstatite structure is found for the small perovskites in figure 2. Superimposed upon the ions like Mg+2 (0.66 A ). cubic group are those compounds that are tetragonal Except for the Ba and Sr vanadates described in (c Ja> 1) or pseudo-tetragonal (c Ja< 1), ferroelectric table12, all other compounds:described in this study and an tiferroelectric, respectively . An area showing 81 t4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 TI Sn Hf Zr Ce U Th Ti Sn HI Zr 1.00.--,---;----'----,'----.--'-'-,--,---,..--,-----, I ' I 0 \ x \ 00 " 1.30 .90 I \ \ I PSE UDOTETRA GONAL TETRAGONAL I RHOMBO- \ ~ £ C ---'--- '\HEDRAL \ / a < I 1.25 ,, ) 1 I , 'y ~.80 I , // , ,/ I \ ,/ ,/ pt+2 Y T ABLE 3. M i xed oxides of the ty pe A +3 B +3 0 3 Toler ance H eat treatment lactor lor 1------,--- Composition StrLl ctm e type Symmetry R eferences and d iSC llS'5io n perov 'l'ernprratul'o Time skite (00) (hr) struc ture a :Mi xed oxides con taining Ah03 1,550 ______P crovs kite ______Rhombohedral b ______Present work. a=3.788 A. «=90°4'. D istor tion so slight that symmetry is assigned m ainly LaAIO , ______on tlte basis oj com parison 'with oth er alumina 0.94 comp ounds. Pero vski tc. ______CaTi0 3 typc ______Naray -Szabo [26]. Perovskite ______Nearly cubio ______Keith and Roy [19] . P erovs kitc. ___ ._._. __ _ Rhombohedral b ______Present work. ,, =3.766 A, « =90°12'. CeAI03______. 91 Pcro vski tc. ______'retragonaL ______Zachariasen [53!. t~:~~;~~~ ;~:: :: Perov :-: kitc. ______U nknown c ______Keith and Roy 19]'0 Pe rOV sk~te ______1 dAI03______1,550 ______{ Rhombohedral b ______Present work. ,, =3.747 A . cx=90023'. .90 Pero vskl te ______RhombohedraL ______Keith and Roy [19]. SmAIO, ______.89 Pcro vski tc. ______Pro bably rhom bo- Keith a nd R oy 119] . P robably has Sltme Iy pe hedral. oj struct"re as La-Ce and NdA103. B i,0 " Ah 0 3 ______.87 P erovskite ______'f etragonal d . ______)laray-Szabo 126] . 1,550 ______P erovskHe plus garneL Orthorhombic (' plus Present work. A pproximately 50 percent oj cubic. each p hase p resent (perovsl:ite p ltase , a=5.176, b=7.355, c=5.307 A). 1,500 ______P erovskite pi us garneL Orthorhombic e plus Present work. A mount oj perovskitepllase has cubic. decreased w ith respect to {Jarnet ph ase . 1,550 ______19 P ervoskite plus garneL Orthorhombic e plus Present work. A mount oj perovskile phase has YAI03______cubic. dec reased witlt res pect to {Jarnet phase. .86 1,835 , ______Garnet plus perovskitc. Cubic plus ortho Present work. Very smalt amount oj ])erov rhombic. e sliite IJh ase 1Jresent. Low ______Garnet solid solution __ Cu bic ______Keith and Roy [19]. High ______YCrO, type ______T etragonaL ______Keith and Roy lI9]. T etra{J onal Y CrO,-type structure lJ robubly is actually an orth orhombic perovskile stmcture (see labte 4). Perovskite. ______CaTiO, type ______;\Iaray-Szabo [26]. In'0 3: Ah 0 3______.82 'rh03 type ______Cllbie ______Kcith and R oy [19]. Fe'0 3: Ah O, ______. 76 Corwldum ______RhombohcdraL ______Keith and Roy [19]. Cr' 0 3:AhO, ______.75 Corundum ______R holll bohedral ______Keith and R oy lI9]. Oa'0 3: Ab 0 3______Coruudum ______Hholll bohedral ______Ah O, ______.75 Keith and Roy [19]. . il 1,300 ______15 Corundulll ______RhombohedraL ______Present work. a=5.13, a =55°J9'. Nlixed ox id es containing GaZ03 P erov skit e ______. Orthorhombic tl ______Prese nt work. Very slight distorlion. a=5·494, LaOaO' ______0.89 b=7.769, c=5.579 A . e~~~:::::::::: :: Perovskitc ______Keith a nd Hoy lI9] . CeOaO' ______. 86 Pcrovski tc ______Keith and Roy [19]. T his compound is prob abty also orthorhombic. P erovskite ______Orthorhombic e ______Prese nt work. Distortion greater than in NdOaO' ______. 85 LaOaO,. a=5.424, b=7.704, c=5.~9 6 A . t~~----:~:::::::::: Perovskitc ______Hhom bohedral or Keith a nd Roy lI9] . monoc li nic. OarneL ______Keith and Hoy [1 9]. Specimen heated to Sm'0 "Oa'0 3____ _ .84 moderate temperature. t:::::::::::::::::: Unknown ______Keith and Roy [19]. Specimen heated to hi ylt temperat"re. 08rneL ______Keith and Roy [19] . Specimen healed to Y,03: Oa'0 3______0.81 moderate temperature. t::::::: :::::: ::::: UnknO\V'Il ______. ______Keitll a nd R oy lI9]. Specimen heated to high temperature. Oa'0 3______.71 Corundum______Hbombohedral ______Keith and Roy [19] . 1\1ixed ox ides conta in ing enOl Perovskite______Cll bic ______)laray-Szabo [26]. Perovskite ______Keith and Roy lI9]. "Slight distortion LaCrO' ______0.88 probably monoclinic. Jt Orthorhombic. Perovskite______Cubic ______Wold and Ward 138]. " P ossibly very slightly ciist.or tcd.'1 P erovskite ______M onoclinic or ortho- Yakel [43]. rhombic. CeCrO' ______.86 Perovsk ite ______Nearlyeubic ______Keith and Hoy [J9]. NdCr03______.85 Perovskite ______R hombohedral or Keith and Roy [19]. Probably ortilOr/lOmbic. monocli nic. SmCrO' ______. 84 YCrO, type ______T etragollaL ______Keit h and R oy [1 9] . r crovski tc______:Molloclinic ______Loo by and Katz [36]. P robably an ortho YCr03______.81 rhom"ic perovsliite oj Ca'i'iO, t//pe. t:::::::::::::::: YCrO, type. ______'l'etragonaL ______Keit h and Roy [19]. All the YCr03-type strU,ctllres are probabt!J or thorhombic perot; skites oj CaTiO, type. F e,O" CI'20, .. ___ _ . 71 Cornndum ______Rhom bohedraL ______Keit h and Hoy lI9]. c.,o, ______. 71 As recei v'cd ______Corundu m______RhombohedraL ______Present work. a=5.S8 A, cx=54°50'. 84 TABLE 3. Nlixed oxides of the type A+3 B +3 0 3- Continued Tole .. · ance ] feai t .. eatment factor fo .. Composiiion Structure type Symmei.. y References and discuss ion pcrov rrimc skite (h .. ) struc Lurca ~ix ed oxidcs co ntaining Fe,O, Perovskite______Orthorhombic e ______Present work. 0=6.545, b=7.85J, <=5.562 A. Pe.. ovskite_ •.. __ .... _. Ca'I'iO, typo ______Naray-Szabo [26J. LaFe0 3______0. 88 Pe .. ovskite .. __ . ____ . ___ ._ .. ______. ___ . ___ _ Keith and Roy [19]. "Ve.. y slight disto .. tion, possibly lllonoclin ic." Orthorhombic. Pcrovskite. ______.. Cubic ______. __ . __ W. L. Roth Ill], Yakel [4 0]. Unit cell twice that oj simple perovskite. Ce1'e0 3______. 86 Perovskite ______P .. obably monoclinic . Keith and Roy [19] . Orthorhombic NdE'e0 3______. 85 YCr03 type .. ______. Tetrago nal ______. ____ _ Kei th and Roy [19J. Probably orlhorhorn/lic perovskite. Sml"eO, _. ______. 83 YC .. 0 3 typo... ______'l'etra;ronaL __ __ . _____ . Keith and Roy [19J . Probably orthorhombic perov.kite. GdFe03______.82 Pe .. ovskite_ .. ______. __ O .. thorhombic ' .. ____ _ Gellc .. 154]. Same space group listed as given by lvIegaw [[5J Jor CaTiO,. P erovskite. ______Orthorhombic e "______Prese nt work. 0=5.279 , b=7.G09, c=5.580.'1. YFeO, ______Largest distortion fronl, cubic symm,etry oj .R I any oj tile 1Jervoskites in the present study. YC .. 0 3 typo ______T etragonaL ______. __ . Kcith and Roy [i9]. / Ill YCr03-t.ype strac C~~'-~~::::::::::: lures are probably orthorhombic perovskites. In,0 3: Fe'0 3______O. i7 '1'1203_____ . _____ ._____ CubiC ______. ______. Keith and R oy [19]. Fe'O' ______· i l Coru ndum _____ ._.__ __ Rh om bohed .. aL ______Keith and R oy [19J . :r. 1i xed ox ides co ntaining SO,0 3 LaSc0 3__ . _.. . _. __ 0. 82 Y Cr03 ______TctragonaL ______Keith and Roy [19J. Probably orthorhombic lJCrOl)skite. CeSc03____ . _____ . · i 9 YCr03__ . _..... _...... TctragonaL ..... _.. _.. Kei th and Roy [19]. Probably orthorhombic perovskile. NdScO, _____ ... __ _ · i8 YCrO' ...... _...... T eiragonaL ...... Keith and Roy [19]. Probably orthorhombic perovskite. Y,O, : Sc,O,_ ...... 75 '1'1, 0 3. ..• _... _...... _. Cubic ...... _._ ...... K eith and Roy I L9]. In, O, : So,O, ...... _ .71 '['1,0 , .... _.. _...... _. C ubic._ ..... _...... _.. K cith a nd R oy [lOj' Sc,O, .... _._ ...... _ .il '1'1,0 ,_._._ .. ___ .. _____ C ubic_...... _.... _. K eit h and R oy [19 . ~1ix cd oxides co ntainin g 1n203 Present work. (£=5.728, b=8.207, c=5.914. A. Keith and Roy [19J. Probably orthorhombic LaI n03.. _.. ... _.. 0. 81 perovskite. f::: ~~~~~~~~:~~~~~ :::::::::: ;~:::~::~c~:_:.:_~~~~ ~~~ ~::~:f::~~~i :~;,~~,~_:: : P ad urow and Sc hustcrius [55J. rhombic). NdI n03...... _ · i 8 1,350______.5 Pcrovskitc ______Orthorhombic e ______Present work. a=6.627, b=8.121 , c=5.891. A. Orthorhombic e ______Smln03_ .. _...... 76 Prese nt work. (£=5.589, b=8.082, c=5.880. A. C ~~~~ ~:::::: : :::::: ____ :~ __ _. ~f;o;~ ~~~~ ~~:::::::::: C ubic._. ______K cit h and Roy [L9J. Probably a mistake on the chart as no mention is made oj th is composi· tion in the teIt of th e 1)('1Jer. '1'1,0, ...... __ ... __ C ubic _____ . ______.. __ _ Y,0 a: ln' 0 3____ .... Padurow a nd Scllllste rius [55J. . 74 {::::::::::::::::::: '1'1,03...... _ C ubic ______. ____ ._ Kcith and Roy [t9J. 1n' 0 3.. ___ ... ___ . .. · il As reccivecL __ .. __ _ '1'1203______Cubic ______Prcsent work. a=10.117. A. Oxides of rare-earth iype L aY 0 3______O. Ii Perovskiic_ . ______Pseudocubic (ortho- P adurowand Sc husierius [55]. rhombic). y ' 0 3______.7l '1'1,0,_._ .. ______Cubie ______. ______. .Iceith and Hoy 119j' L a203: Sm20 3______· i5 '1'1,0,_ .... ______Cubic ______• ____ . __ . Keith and Roy 119. Sm'03______'1'1,0, .. _.. ___ . ______Cubic ______.. __ Keith and Roy [19 . ='Id , 0 3______· i1 · i l As received ______La20 3______H exagonaL ______Present work. a =3.818, c=5.994. A. CO' 0 3______· i 1 La'03. ___ ._. ______Il exagon al. .. ______Keith and Roy [19J. La203______. il 1,200 .. ______La20 3______HexagonaL ______Prese nt work. a=3.405, c=6.13g. A . " U n les~ otherwise indicated, rad ii of ihe ions are iaken fr om Ahrens [30J. b This type of rhombohed ral d istortion of the perovskite siruciure is similar io that reported by Askhao, F ankuchen, and Ward [07] for LaCo0 3. r A symmetrical doublet was report.ed for the (111) refl ection. 'rhis doublet was also observed in present work for powdered m at.rriul ; however, intact specimen showed twice the in tenSity for lower-angle pea k, indicating rhombohedral symmetry wit ll <»90°. d Thc siructure sugges ted for BiAlO3 is similar to t hat of PbSnO,. As the Jattcr compound does not exist, the compound BiAIO, cannot be accepted without further experimental confirmation. _ e 'Phis structure js related to cubic perovskitc as follows: a~ ,j2a', /)~2a', c~ -J2a' . f Tempcraturcs above 1,550° C werc obtaincd by cmploying a gas-fircd co mmercial furnace capable of practlcal opcration up (0 aboui 1,850° C. 85 b. YAIOa Y20 a:A120 a heated at 1,835° C 5 for 1 hI' contained The compound YAIOa was assigned to a new struc even less of the perovskite structure. It seems that ture type, YCrOa, by Keith and Roy [19]. How attainment of equilibrium between the garnet and ever, the compound YCrOa has been called a perov perovskite structures in Y AIOa is a slow process, skite by Looby and Katz [36]. The X -ray pattern and one of these structures might be only a meta for the YCrOa-type structure in YAIOa specimens can stable phase and have no true cquilibrium position. definitely be indexed as a perovskite type, as can be A detailed analysis of the phase equilibria throughout seen in table 4, and is similar in all respects to the the Y20 a-A120 a system would be necessary to answer this problem completely. CaTi03-type structure. This structure has the addi tional advantage of having a much smaller unit cell. Actually the distortion from a cube is no greater 4.4. Classification of ABOa Compounds Containing than that found in CaZrOa or CaSnOa and does not Double Oxides of Trivalent Ions deserve a new structure-type designation. There fore, the orthorhombic-type structure, originally As very little is known quantitatively about the assigned to CaTiOa, may now be considered to be polarizability values of the trivalent ions in perov the more appropriate structure for the compounds skite compounds, this factor has not been used in referred by Keith and Roy [19] to the YCrOa type. the present work to describe the classification of Y AIOa has been reported to form a garnet (solid these structmes. All the compounds studied are solution) type structure by Kcith and Roy [19] who indicated in figme 5 according to the radii of the state that the garnet structure is a low-temperature constituent A and B ions. As a complete diagram form and the perovskite (YCrOa) type structure is the would list each compound twice, the B ion, for con high-temperature form. This observation does not venience, is always taken as the smaller ion. It can agree with the evidence found in the present study be seen from this diagram that all of the compounds (see table 4). A 1:1 mixture of Y 20 a and A120 a in the upper left of the diagram, large A +3 and small heated for 1 hI' at 1,5 00 ° C contained about 50 per B +a ions, form perovskite-type structmes. This cent of the garnet-type structure and 50 percent of group is followed by a belt of T120a-type structures, the perovskite-type structure. vVhen this specimen most or all of which probably represent solid solutions was reheated at 1,550° C for 19 hI' the amount of the rather than true compounds. The lower left corner, perovskite-type structure had greatly decreased containing small ions in both the A and B positions, relative to the garnet type. A second specimen of shows a field of corundum (or ilmenite) type struc tures, probably solid solutions. The upper-right corner, in which both A and B are large ions, contains T ABLE 4. X-my powder dip'mction data for YAlOa a field of La20 a-type structmes, probably solid solu tions rather than compounds. This diagram does Keith and Roy Present work b not differ appreciably from that of Keith and Roy [19] • [19] . However, the perovskite field is enlarged to I accept all those compounds classified by Keith and d IIkl d hkl Relative Roy as of the YCr03 structure. In addition, a line intensity (' is drawn to separate the field of rhombohedral perov Observed Calculated ------skites from the rest of the perovskite compounds, A A A which apparently all have the CaTiOa-type structme. 002 4.24 110 4.23 4.23 II It should be emphasized here that no compounds of 102 3.70 101 3.70 3.71 34 200 3.68 020 3.68 3.68 22 the A+3B +30 3 type are known to have a simple cubic 201 3. 36 (Ah03) ------112 3.32 III 3.31 3.3l 23 perovskite-type structure. One discrepancy in figme 5 might be found for the 003 2.89 /i121 2.90 ------5 103 2.67 002 2.65 2.65 24 LaCo03 compound reported by Askhan, Fankuchen, 212 2. 62 121 2.61 2.61 100 220 2.59 200 2.59 2.59 32 and Ward [37] . Co+a is listed by Ahrens [30] as ]13 2.51 012 2.50 2.50 12 having a radius of 0.63 A, essentially the same as ------J02 2.38 2.36 5 that of Cr+3 . This would place the compound 222 2.22 21l 2.220 2.2J8 8 2 13 2.16 022 2. 154 2.1 52 21 L aCo03 wcll within the field of orthorhombic pcrov 302 2. 12 220 2.082 2. ]]6 9 skites, instead of in the rhombohedral field as it 312 2.05 131 2. 048 2.045 9 should be. A possible explanation of this discrep 114 1. 996 122 ------I. 99 ------321 1. 972 221 ------I. 97 ------ancy, other than assuming incorrect radii or resorting 204 1. 859 202 1. 854 1. 853 26 to nonstoichiometry, is the possibility of having large 400 I. 845 040 1. 843 1. 839 2l 214 I. 806 032 I. 799 1. 801 14 polarizability in the lanthanum cobaltate compound. 331 1. 694 ------The compound occW"ring in the specimen of Ce02: 224 I. 655 141 I. 651 1. 647 10 Ti02, mentioned earlier, can be explained by this 304 1. 640 ------4 3J4 1. 605 ------diagram if it is assumed that both Ce+ and TiH have been reduced to the trivalent state. Ti+3 has a radius a This pattern is actually from a speci men of 3Y, 0 ,,5Ah03 stated of approximately 0.76 A [30] and the compound as having tbe YCr03 type of structure similar to the 1:1 specimen. b X-ray diffraction lines due to tbe garnet solid-solution phase have been omitted. , Observed intensity of tbe difraetion peaks relative to the stron g ' T emperatures above 1,550° 0 were obtained by employing a gas-fired com est peak. m ercial furnace capable of practical operation up to about 1,850° C. 86 -- ---, two . Sup erimposed on the fi eld of cubic compou nd s d' is an area of ferrociectric structures. A three 1.20 I RHOMBOHED- ( dirnensional graph, using the polarizability of the RAL 0 I DO · 0 La" 1.10 PEROVSKnE divalent ions as Lhe third dimension, shows that thi 0 I o • I I I area is really a projection onto a basal plane of a 0 I 0-- · 0 / / , X.. I volume in space encompassing ferroelectric and anti ql.OO . 0 / / / .n ORTHORHOMBIC / ferroelectric compounds and solid solutions. Z 0 )!(J ,./ A two-dimensional plot using ionic radii has also fl)f!. ,90 , / +« / -- been constructed for double oxides of the trivalenL ------ions. This graph shows perovskite compounds i5 .80 88 --_.. _-