Structure determination of A2M3+TaO6 and A2M3+NbO6 ordered perovskites: octahedral tilting and pseudosymmetry.
Structure determination of A2M3+TaO6 and A2M3+NbO6 ordered perovskites: octahedral tilting and pseudosymmetry.
复制标题
DOI：
10.1107/s0108768106002448
复制
发表时间：
2006-06
期刊：
影响因子：
--
通讯作者：
P. W. Barnes;M. Lufaso;P. Woodward
中科院分区：
文献类型：
--
作者：
P. W. Barnes;M. Lufaso;P. Woodward
根据中子和X射线粉末衍射数据确定了六种A(2)M(3+)M(5+)O(6)有序钙钛矿的室温晶体结构。 Ba(2)YNbO(6)采用亚里斯多型高对称立方结构（空间群Fm\overline 3m，Z = 4）。其余五种化合物的对称性因八面体倾斜扭曲而降低。在 Sr(2)CrTaO(6) 和 Sr(2)GaTaO(6) 中观察到八面体绕 c 轴的异相旋转，这将对称性降低到四方（空间群 = I4/m，Z = 2，Glazer 倾斜系统 = a(0)a(0)c(-)）。与 GdFeO(3) 中看到的类似的八面体倾斜发生在 Sr(2)ScNbO(6)、Ca(2)AlNbO(6) 和 Ca(2)CrTaO(6) 中，这将对称性降低为单斜晶系（空间群 P2(1)/n，Z = 2，Glazer 倾斜系统 = a(-)a(-)c(+)）。 Sr(2)MTaO(6) (M = Cr, Ga, Sc) 化合物的晶胞尺寸是高度伪立方的。 Ca(2)AlNbO(6) 和Ca(2)CrTaO(6) 的晶胞尺寸是强伪斜方晶系。这种高度的赝对称性使空间群分配和结构确定变得复杂。还报告了根据 X 射线粉末衍射数据确定的另外 13 种组合物的空间群对称性、晶胞尺寸和阳离子排序特征。对 32 A(2)MTaO(6) 和 A(2)MNbO(6) 钙钛矿晶体结构的分析表明，通常八面体倾斜系统与容差因子密切相关。
The room-temperature crystal structures of six A(2)M(3+)M(5+)O(6) ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba(2)YNbO(6) adopts the aristotype high-symmetry cubic structure (space group Fm\overline 3m, Z = 4). The symmetries of the remaining five compounds were lowered by octahedral tilting distortions. Out-of-phase rotations of the octahedra about the c axis were observed in Sr(2)CrTaO(6) and Sr(2)GaTaO(6), which lowers the symmetry to tetragonal (space group = I4/m, Z = 2, Glazer tilt system = a(0)a(0)c(-)). Octahedral tilting analogous to that seen in GdFeO(3) occurs in Sr(2)ScNbO(6), Ca(2)AlNbO(6) and Ca(2)CrTaO(6), which lowers the symmetry to monoclinic (space group P2(1)/n, Z = 2, Glazer tilt system = a(-)a(-)c(+)). The Sr(2)MTaO(6) (M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. Ca(2)AlNbO(6) and Ca(2)CrTaO(6) have unit-cell dimensions that are strongly pseudo-orthorhombic. This high degree of pseudosymmetry complicates the space-group assignment and structure determination. The space-group symmetries, unit-cell dimensions and cation ordering characteristics of an additional 13 compositions, as determined from X-ray powder diffraction data, are also reported. An analysis of the crystal structures of 32 A(2)MTaO(6) and A(2)MNbO(6) perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.