Electron Counting and High-Pressure Phase Transformations in Metal Hexaborides

Pressure-induced structural transitions of the alkaline earth hexaborides, CaB6, SrB6, and BaB6, are studied theoretically using electron counting rules and density functional theory calculations. We demonstrate the applicability of gas-phase borane electron counting methods to solid-state metal borides under pressure and validate the assumptions of the rules by density functional theory (DFT) calculations. All three compounds share ambient-pressure and high-pressure structures, but BaB6 differs from CaB6 and SrB6 at intermediate pressures. The unique BaB6 phase is shown to break electron counting rules, while all other phases obey them. This anomaly is resolved by DFT, which reveals B–Ba covalency and unusual B–B π bonding under pressure. The relationships between structure and bonding can help us to understand the exotic behavior of lanthanide hexaborides and design new borides with desirable properties. Developing electron counting procedures for solids will enhance materials discovery efforts with chemical intuition.

本研究采用电子计数规则（electron counting rules）与密度泛函理论（density functional theory, DFT）计算方法，从理论层面系统探究了碱土金属六硼化物CaB₆、SrB₆及BaB₆在压力诱导下的结构相变行为。本研究证实，气相硼烷电子计数规则可推广应用于压力环境下的固态金属硼化物体系，并通过密度泛函理论计算验证了该规则的核心假设。三种目标化合物均具备常压与高压稳定结构，但在中等压力区间内，BaB₆的结构演化行为与CaB₆、SrB₆存在显著差异。独特的BaB₆相被证实违反了电子计数规则，其余所有晶相均符合该规则的要求。密度泛函理论计算阐明了这一反常现象：压力下体系中存在B-Ba共价相互作用与非常规B-B π键。结构与键合之间的构效关系，有助于理解镧系六硼化物的奇异物理行为，并指导设计具备目标性能的新型硼化物材料。建立适用于固态体系的电子计数方法，可借助化学直觉加速新材料的发现与开发进程。