Lin Qisheng, Corbett John D
Department of Chemistry, Iowa State University, Ames, Iowa 50010, USA.
Inorg Chem. 2008 Sep 1;47(17):7651-9. doi: 10.1021/ic800694j. Epub 2008 Aug 2.
Two crystalline approximants (ACs) and their corresponding icosahedral quasicrystal (i-QC) are obtained in the Ca-Au-Ga system through conventional solid-state exploratory syntheses. Single crystal structural analyses reveal that the 1/1 AC, Ca 3Au x Ga 19- x ( x = approximately 9.3-12.1) [ Im3, a = 14.6941(6)-14.7594(6) A], has the empty cubes in the prototypic YCd 6 (= Y 3Cd 18) now fully occupied by Ga, resulting in a 3:19 stoichiometry. In parallel, the distorted cubes in the 2/1 AC, Ca 13Au 57.1Ga 23.4 [ Pa3, a = 23.9377(8) A] are fully or fractionally occupied by Ga. The valence electron count per atom ( e/ a) for the 2/1 AC (1.64) is smaller than that over the 1/1 AC composition range (1.76-2.02), and the e/ a of the Ca 15.2Au 50.3Ga 34.5 i-QC, 1.84, is somewhat distant from typical values for Tsai-type i-QCs ( approximately 2.0). Comparisons of the gallium results with the corresponding In phases suggest that the structural differences result mainly from size rather than electronic factors. The 1/1 and 2/1 appear to be thermodynamically stable on slow cooling, as usual, whereas the i-QC isolated by quenching decomposes on heating at approximately 660 degrees C, mainly into 2/1 AC and Ca 3(Au,Ga) 11. Calculations of the electronic structure of 1/1 AC suggest that the Fermi sphere-Brillouin zone interactions remain important for the Ca-Au-Ga i-QC.
通过传统的固态探索性合成方法，在Ca-Au-Ga体系中获得了两种晶体近似物（ACs）及其相应的二十面体准晶体（i-QC）。单晶结构分析表明，1/1的AC，Ca₃AuxGa₁₉₋ₓ（x≈9.3 - 12.1）[Im3，a = 14.6941(6) - 14.7594(6) Å]，在原型YCd₆（= Y₃Cd₁₈）中的空立方体现在被Ga完全占据，从而得到3:19的化学计量比。同时，2/1的AC，Ca₁₃Au₅₇.₁Ga₂₃.₄ [Pa3，a = 23.9377(8) Å]中的扭曲立方体被Ga完全或部分占据。2/1的AC的每个原子的价电子数（e/a）为1.64，低于1/1的AC组成范围内的值（1.76 - 2.02），并且Ca₁₅.₂Au₅₀.₃Ga₃₄.₅ i-QC的e/a为1.84，与蔡氏型i-QC的典型值（约2.0）有些不同。将镓的结果与相应的铟相进行比较表明，结构差异主要源于尺寸而非电子因素。与往常一样，1/1和2/1在缓慢冷却时似乎是热力学稳定的，而通过淬火分离出的i-QC在约660℃加热时分解，主要分解为2/1的AC和Ca₃(Au,Ga)₁₁。对1/1的AC的电子结构计算表明，费米球 - 布里渊区相互作用对Ca-Au-Ga i-QC仍然很重要。
