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131
O R D E R I N G O F OCTAHEDRALLY COORDIKATED CATION POSITION
ORDERING OF THE OCTAHEDRALLY COORDINATED CATION POSITION IN THE PEROVSKITE STRUCTURE BY FRANCIS GALASSO AND WILDADARBY United Aircraft Corporation, Reaearch Laboratories, East Hartford, Connecticut Received August 14, 1961
An X-ray analysis of compounds with the general formula Ba(M3+0.&b5+~.~)0~, where R P + is a rare earth, indium or iron ion, has shown that the critical percentage difference in radii necessary for ordering of the ME+and niobium ions in the B sites of the perovskite structure lies between 7 and 17%. Of significance, this value found for ordering encompasses the 15% which has been used as an indication of the maximum percentage difference in the sizes of atoms or ions involved in either solid solution formation or isomorphous substitution.
Introduction Oxides with the perovskite structure are represented by the formula AB03 where A is a large metal cation close packed in layers with oxygen ions, and B is a smaller metal ion situated in an octahedrally coordinated hole between the closepacked layers. As postulated in ref. 1, if two ions are present in the B position, ai1 ordered distribution of the B site ions is most probable when large differences exist in either their charges or ionic radii. This hypothesis is validated qualitatively in Table I, which illustrates the arrangement of B ions in some perovskite-type compounds with variations in both size and charge differences. A review of the published literature, however, indicates insufficient data are available to permit a quantitative de termination of the critical differences in charges or radii necessary for transition from a random io an ordered arrangement of the B position ions. Consequently, studies have been initiated a t the United Aircraft Research Laboratories in an effort to determine these critical size and charge differences. The purpose of this report is to present the results of a study of ordering of the B position in a series of perovskite-type compounds whose diffgrence in ionic radii varied between 0.05 and 0.45 A., while the charge difference mas held constant. TABLE I ORDERED A N D DISORDERED PEROVSKITE
CO&lPOUNDS
Diff. in Diff. in ionic radii chargeof of the B B ions ions, B.6
Arrangement of B
Compound
Ba( ilIg2+o.bW6+ 0 . 6 ) 0 3 Ba(Zn2*0.33Nb6+0.Bi)03 Ba(Fe3*0.sTa6+o0.5)O3 Ba( Sr2+o.33Ta5+0.6, 1 0 3 Ba(La3+0.6'Ca6+~.~)0~ Ahrens radii values used.
4 3 2 3 2
0.05 0.05 0.04 0.44 0.46
Ordered2 Random' Random' Ordered3 Ordered4
+
compounds selected
for study are represented by the formula Ba( Ma+~.&Tb6+o.,)08 where M 3 + k a rare earth ion (La3+through Lu3+),Ins+ or
Fe3+. Since the difference in ionic radii of the B ions varied from 0.45 t o 0.05 A. in this series of compounds, a transition from an ordered to a disordered arrangement of the B sites was expected as the ionic radius of M approached that of niobium. (1) F. Galasso, L. Katz and R. Ward, J . Am. Chem. Soc., 81, 820 (1969).
(2) E. G. Steward and H. P. Rooksby, Acta Cryst., 4,503 (1951). ( 3 ) F. Gala,sso and L. Katz, J . Am. Chem. Soc.. 83,2830 (1961). (4) I,. Brixner, ibid., 80, 3214 (1958).
+
Discussion of Results The results of the Research Laboratories' study reveal that there are weak superstructure lines due to ordering in the X-ray photographs of the compounds prepared with the general formula Ba(Mo5Xbo.5)03,when M was a rare earth or indium ion. In some cases, the ordering lines were diffuse; the diffuseness probably was caused by the small size of the ordered domains. A logical ordered structure for these compounds where the TABLE I1 STRUI :TURE CALCULATIOKS FOR Ba(Ndo.5Nbo.dO4 Sin* 0 (obsd.)
Sins 0 (calcd.)
220 311
0.0244 .0647 ,0893
0.0243 .0649
222
.0972
,0892 ,0973
400 331 422 440 620 444 642
,1296 .1540 .1943 ,2594 .3241 .3885 .4536
,1296 ,1541 .1946 ,2595 ,3244 .3893 .4542
hlcl
111
Experimental Procedure Preparation of Compounds.-The
I n all cases the compounds were prepared by mixing barium carbonate, niobium pentoxide, and the appropriate indium, iron or rare-earth sesquioxide according to the formula 4BaC03 NbnOs h&o3--+ 4Ba(Mo 5Nb0.5)03 f 4COz and heating the mixture in Leco boats a t 1200" for 24 hours. It should be noted that only those rare earth sesquioxides that are stable in air a t the preparation temperatures were used in the mixtures. X-Ray Analysis.-Powder X-ray diffraction photographs were taken of the compounds formed using a 57.3 mm. radius Phillips X-ray powder camera and high intensity copper Ka radiation with settings of 50 kv. and 40 ma. In order to observe the superstructure lines, the photographs were exposed from 4 to 7 hours and in the case of Ra(Ino~Nbo.5)03, for 24 hours. The X-ray patterns were read, indexed, and the cell sizes determined by extrapolation of a0 US. 1/2(cos*~/sine-t- cos2e/e) to e = 90O.5 High temperature X-ray diffractometer tracings up to 1000° were made of B a ( N d 0 . & b ~ . ~ )and 0 ~ Ba(Luo.sNbo.dOa using a Norelco diff ractometer with an attached Tem-Pres heater. These specimens were selected for study because they represented the largest and smallest differences in ionic radii of the B ions in the cubic barium-rare earth niobates. Since no order-disorder transformation of the B sites in either of these compounds was found (see Fig. l), further high temperature X-ray studies of the barium-rare earth niobate Compounds with intermediate size differences of the B ions were not attempted a t this time.
rx
10-6
(calcd.
4.1 122.5 1.3 3.4 47.0 0.9 57.1 29.9 29.5 8.8 34.5
r
(obsd.)
w
vs w W S
vw S
TCI M
W M
(5) J. B. Nelson and D. P. Riley, Proc. Phys. Soc. (London), 61, 160 (1945).
132
FRANCIS GALASSO AND WILDADA~ZBY
Vol. G6
TABLE I11 STRUCTURE DATAFOR Ba( M a +0.6Nb5*0.~)Oa COMPOUNDS Cell size A. a = 8.607 c = 8.690
Compound
Ba(Lao.i”o.dOa Ba( Nd0.6Nbo.dOa Ba(Smo.sNbo.s)Os Ba( E u d bo. E.)0 3 Ba( Gd0.6Nbo.dOp WDYO.WO,S)O~ Ba(Hoo.~Nb0,~)03 Ba(Er0.6Nb0.dOa Ba(’irno.sNbo.dOs WYb0.6Nbo.dOa Ba(Luo.:Nbo.dOl €34Ino.&bo. )03 Ba(Feo,6Nbo.dOa a From Brixner.6 * Ahrens radii values
Cell sizes A.0
8.540 8.518 8.507 8.496 8.437 8.434 8.427 8.408 8.374 8.364 8.279 4.057 used.
T E M P E R A T U R E . ‘C
Fig. 1.-Barium-rare
earth niobates: thermal expansion.
tu70 B ions are present in equal proportions is one suggested by Steward and Rooksby, in which the t v o B ions alternate so that the perovskite unit cell edge (-4 A.) has to be doubled. Therefore, all of the X-ray photographs were indexed using the double cubic perovskite cell parameter, except for Ba(La0,~hTbo,~)03, which also was tetragonally distorted and Ba(Feo,&b0.~)03, in which the iron and niobium ions were not ordered. Table I1 gives the indexing data as well as observed and calculated intensities; the latter were computed for Ba(Ndo.bNbo,5)03,assuming an ordered arrangement of Nd and Nb ions. The data indicate the ordered perovskite structure for these compounds is probably a correct one. This is in contrast to the observations which were reported by Brixner6 while this study was in progress. Brixner indicates that in these barium-rare earth niobates the rare earth (6) L. Brixner, J . Inorg. 6: Nuclear Chem., 16,352 (1960).
4.298 4.337 4.277 4 248 4.234 4.242 4.224 4.216 4.208 4.201 4.192 4.187 I
Arrangement of B ions
Ordered Ordered Ordered Ordered Ordered Ordered Ordered Ordered Ordered Ordered Ordered Ordered Random]
Diff. in ionic radii % Diff. in ionic of B ions, A . b radii of B ions
0.45 .35 .31 .29 .28 .23 .22 .20 .18 .17 .IF
.12 .05
65
51 45 42 41 33 32 29 26 25 23 17 7
and niobiouni ions are arranged randomly in the B sites. Since the Research Laboratories’ data show that the lines due to ordering in the X-ray photographs are weak, it is conceivable that Brixner may not have observed them in his investigation. It should be noted that his range of exposure times for X-ray photographs (2-4 hr.) is less than that used in this investigation. I n Table I11 the cell sizes and the differences and percentage differences in the ionic radii of the B ions a t room temperature for thc prepared compounds are presented. These data indicate that the critical percentage difference in ionic radii between B ions in Ba(Mo.K~bo.a)03 compounds which causes ordering lies between 7 and 17%. The inability to determine more accurately the critical size difference for ordering for these compounds is due to the limitation of available trivalent ions with ionic difference between 0.05 and 0.12 8. However, the results of this investigation are particularly interesting in view of the observation of XIason’ that a wide range of isomorphous substitution in ionic compounds is possible if the radii of the substituting and substituted ions do not differ by more than 15%, and Hume-Rothery’s rule on atomic size factor which states that atoms which differ in diameter by more than 15% should form limited solid solutions. ,4lthough neither Mason’s statement nor Hume-Rothery’s rule applies directly t o order-disorder phenomena, it appears significant that this value of 15% also falls into the critical size range required for the ordering of B ions as found in this study. (7) B. Mason, “Principles of Geochemistry,” John Wiley and Sons, Inc., New York, N. Y.,1958.
