Häussermann Ulrich, Amerioun Shahrad, Eriksson Lars, Lee Chi-Shen, Miller Gordon J
Department of Inorganic Chemistry, Stockholm University, S-10691 Stockholm, Sweden.
J Am Chem Soc. 2002 Apr 24;124(16):4371-83. doi: 10.1021/ja012392v.
This work presents a detailed, combined experimental and theoretical study on the structural stability of s-p bonded compounds with the BaAl4 structure type (space group I4/mmm, Z = 2) as part of a broad program to investigate the complex questions of structure formation and atomic arrangements in polar intermetallics. From ab initio calculations employing pseudopotentials and a plane wave basis set, we extracted optimized structural parameters, binding energies, and the electronic structure of the systems AeX(III)4, AeX(II)2X(IV)2, AeX(II)2X(III)2 (Ae = Ca, Sr, Ba; X(II) = Mg, Zn; X(III) = Al, Ga; X(IV) = Si, Ge). For all systems we found a pronounced pseudo-gap in the density of states separating network X42- bonding from antibonding electronic states that coincides with the Fermi level for an electron count of 14 electrons per formula unit, the optimum value for stable BaAl4-type polar intermetallics. However, the synthesis and structural characterization (from X-ray single crystal and powder diffraction data) of the new compounds AeZn2-Al2+, AeZn2-deltaGa2+delta (Ae = Ca, Sr, Ba; delta = 0-0.2) and AeMg0.9Al3.1, AeMg1.7Ga2.3 (Ae = Sr, Ba) manifested that electron deficiency is rather frequent for BaAl4-type polar intermetallics. The site preference for different "X" elements in the ternary systems was quantified by calculating "coloring energies", which, for some systems, was strongly dependent on the size of the electropositive Ae component. The Ae2+ cations decisively influence the nearest neighbor distances in the encapsulating polyanionic networks X4(2-) and the structures of these networks are surprisingly flexible to the size of the Ae component without changing the overall bonding picture. A monoclinically distorted variant of the BaAl4 structure occurs when the cations become too small for matching the size of encapsulating X4(2-) cages. An even larger size mismatch leads to the formation of the EuIn4 structure type.
作为研究极性金属间化合物结构形成和原子排列复杂问题的广泛计划的一部分，本工作对具有BaAl4结构类型（空间群I4/mmm，Z = 2）的s-p键合化合物的结构稳定性进行了详细的实验与理论相结合的研究。通过使用赝势和平面波基组的从头算计算，我们提取了AeX(III)4、AeX(II)2X(IV)2、AeX(II)2X(III)2体系（Ae = Ca、Sr、Ba；X(II) = Mg、Zn；X(III) = Al、Ga；X(IV) = Si、Ge）的优化结构参数、结合能和电子结构。对于所有体系，我们发现态密度中存在一个明显的赝能隙，它将网络X42-键合与反键电子态分开，对于每个化学式单元有14个电子的电子计数，该赝能隙与费米能级重合，这是稳定的BaAl4型极性金属间化合物的最佳值。然而，新化合物AeZn2-Al2+、AeZn2-δGa2+δ（Ae = Ca、Sr、Ba；δ = 0 - 0.2）以及AeMg0.9Al3.1、AeMg1.7Ga2.3（Ae = Sr、Ba）的合成和结构表征（来自X射线单晶和粉末衍射数据）表明，对于BaAl4型极性金属间化合物，电子缺陷相当常见。通过计算“着色能量”对三元体系中不同“X”元素的位点偏好进行了量化，对于某些体系，“着色能量”强烈依赖于正电性Ae组分的大小。Ae2+阳离子决定性地影响包封多阴离子网络X4(2-)中的最近邻距离，并且这些网络的结构对于Ae组分的大小具有惊人的灵活性，而不会改变整体键合情况。当阳离子变得太小而无法匹配包封X4(2-)笼的大小时，会出现BaAl4结构的单斜畸变变体。更大的尺寸不匹配会导致EuIn4结构类型的形成。
