Peterson Gordon G C, Hilleke Katerina P, Lotfi Sogol, Wang Fei, Zurek Eva, Brgoch Jakoah
Department of Chemistry, University of Houston, Houston, Texas 77204, United States.
Department of Chemistry, The State University of New York at Buffalo, Buffalo, New York 14260, United States.
J Am Chem Soc. 2023 Oct 4;145(39):21612-21622. doi: 10.1021/jacs.3c07936. Epub 2023 Sep 21.
The physical properties of solid-state materials are closely tied to their crystal structure, yet our understanding of how competing structural arrangements energetically compare is limited. In this work, we explore how small differences in composition affect the structure in the La(AuGe) series of compounds, which comprises four unique structure types between LaGe and LaAu. This family includes the previously unknown AlB-type compound with stoichiometry La(AuGe) as well as La(AuGe), an intergrowth of the AlB and ThSi structure types. We then study the chemical forces driving the structure changes and use phonon band structure calculations and DFT-Chemical Pressure to evaluate atomic-size effects. These calculations show that the parent AlB structure type is disfavored in Au-rich compounds due to soft atomic motions along the axis. The instability of AlB-type LaAuGe is confirmed by the presence of imaginary modes in the phonon band structure that correspond to a "puckering" of the hexagonal AlB-type lattice, resulting in the experimentally observed LiGaGe structure type. The impact of size effects is less clear for Au-poor compositions; instead, twisting the AlB structure type to form the ThSi type opens a pseudogap at the Fermi level in the electronic density of states. This investigation demonstrates how crystal structure in solid-state materials can be compositionally tuned based on balancing size and electronics when multiple structure types are in close thermodynamic competition.
固态材料的物理性质与其晶体结构密切相关，然而我们对相互竞争的结构排列在能量上如何比较的理解却很有限。在这项工作中，我们探索了组成上的微小差异如何影响La(AuGe)系列化合物的结构，该系列化合物在LaGe和LaAu之间包含四种独特的结构类型。这个家族包括化学计量比为La(AuGe)的先前未知的AlB型化合物以及AlB和ThSi结构类型共生的La(AuGe)。然后，我们研究驱动结构变化的化学力，并使用声子能带结构计算和密度泛函理论 - 化学压力来评估原子尺寸效应。这些计算表明，由于沿c轴的软原子运动，在富金化合物中母体AlB结构类型不受青睐。AlB型LaAuGe的不稳定性通过声子能带结构中对应于六方AlB型晶格“褶皱”的虚模的存在得到证实，从而导致实验观察到的LiGaGe结构类型。对于贫金成分，尺寸效应的影响不太明显；相反，将AlB结构类型扭曲形成ThSi类型会在电子态密度的费米能级处打开一个赝能隙。这项研究表明，当多种结构类型处于紧密的热力学竞争中时，固态材料中的晶体结构如何基于尺寸和电子学的平衡进行成分调整。
