Department of Chemistry and Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, Iowa 50011, USA.
Inorg Chem. 2011 Dec 19;50(24):12714-23. doi: 10.1021/ic201840q. Epub 2011 Nov 22.
A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La(5-x)Y(x)Si(4) (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La(5-x)Y(x)Si(4) phases with x ≤ 3.0 exhibit the tetragonal Zr(5)Si(4)-type structure (space group P4(1)2(1)2, Z = 4, Pearson symbol tP36), which contains only Si-Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd(5)Si(4)-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si-Si dimers, whereas Y(5)Si(4) forms the monoclinic Gd(5)Si(2)Ge(2) structure (space group P2(1)/c, Z = 4, Pearson symbol mP36), which exhibits 50% "broken" Si-Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La(5-x)Y(x)Si(4) series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si-Si bonding in these phases.
采用高温方法制备的 La(5-x)Y(x)Si(4)（0 ≤ x ≤ 5）系列的晶体结构研究和理论分析，以及 La 和 Y 位的结构和位置偏好。在室温下，富 La 的 La(5-x)Y(x)Si(4 相，其中 x ≤ 3.0，具有四方 Zr(5)Si(4)型结构（空间群 P4(1)2(1)2，Z = 4，Pearson 符号 tP36），仅包含 Si-Si 二聚体。另一方面，富 Y 的 x = 4.0 和 4.5 相采用正交 Gd(5)Si(4)型结构（空间群 Pnma，Z = 4，Pearson 符号 oP36），也具有 Si-Si 二聚体，而 Y(5)Si(4)形成单斜 Gd(5)Si(2)Ge(2)结构（空间群 P2(1)/c，Z = 4，Pearson 符号 mP36），其中 50%的 Si-Si 二聚体“断裂”。讨论了四方、正交和单斜结构之间的局部和远程结构关系。从不对称单位中 La 或 Y 原子的三个独立位置的单晶 X 射线衍射研究的精修揭示了这些元素的部分混合，并且这些元素的偏好明显不同。用于研究 La(5-x)Y(x)Si(4)系列中 La/Y 位偏好和结构趋势的第一性原理电子结构计算表明，长程和短程结构特征主要由原子尺寸控制。然而，La 5d 和 Y 4d 轨道对电子态密度曲线产生了明显但微妙的影响，并影响了这些相中 Si-Si 键的特性。
