THE TWIN STRUCTURE OF LA2TI2O7 - X-RAY AND TRANSMISSION ELECTRON-MICROSCOPY STUDIES

被引:87
作者
SCHMALLE, HW
WILLIAMS, T
RELLER, A
LINDEN, A
BEDNORZ, JG
机构
[1] UNIV ZURICH,INST INORGAN CHEM,CH-8057 ZURICH,SWITZERLAND
[2] UNIV ZURICH,INST ORGAN CHEM,CH-8057 ZURICH,SWITZERLAND
[3] IBM CORP,DIV RES,ZURICH RES LAB,CH-8803 RUSCHLIKON,SWITZERLAND
来源
ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE | 1993年 / 49卷
关键词
D O I
10.1107/S010876819200987X
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
La2Ti2O7, M(r) = 485.613, monoclinic, P2(1), a = 7.812 (2), b = 5.5440 (7), c = 13.010 (2) angstrom, beta = 98.66 (1)-degrees, V = 557.0 (4) angstrom3, Z = 4, T= 298 K, D(x) = 5.790 Mg m-3, lambda(Mo Kalpha) = 0.71073 angstrom, mu = 17.86 mm-1, F(000) = 856. The crystals are twinned. The twin I and twin II intensities are superimposed in reciprocal-lattice layers with h even. The a* and b* axes of the twin components run antiparallel. The angle between a(twin I)* and a(twin II)* is 17.12-degrees. The twinning operation has been identified to be a mirror plane perpendicular to the c* direction affecting only the oxygen positions in the structure: La and Ti atoms lie on an orthorhombic sublattice unchanged by the twinning. The structure refined on Absolute value of F 2 to R = 0.040, wR = 0.085 for 7972 observed reflections with I > 3 sigma(I). The refinement was carried out by inclusion of the contributions from the two twin elements simultaneously, with the use of appropriate matrices to relate the atomic coordinates and reflection indices of each of the twin elements. The twin volume fraction was also refined and gave alpha = 0. 1 108 (3) as the volume fraction of twin 1. The enantiomorphs in twin I and twin II have been shown to be opposite by refinement, on Absolute value of F, of the enantiopole, or Flack's x, parameter against reflection data that had been separated into separate sets for each twin element. Subsequently, several refinements, on Absolute value of F, of the twinned data set with the use of all possible combinations of enantiomorphs of each twin element yielded the best R/wR values (0.040 and 0.056) when the twin components contained opposite enantiomorphs. The general features of the structure determined by Gasperin [Acta Cryst. (1975), B31, 2129-2130] could be confirmed: the twins comprise distorted (4 + 1 + 1) TiO6 octahedra sharing vertices to form infinite perovskite-like layers three octahedra thick, bound by crystallographic shears in the a direction. La atoms occupy distorted perovskitic A sites and interlayer sites. La2Ti2O7 is a member of an homologous series of A(n + 1)B(n + 1)O3n + 5 (0 less-than-or-equal-to n less-than-or-equal-to infinity) compounds with n = 3.
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页码:235 / 244
页数:10
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