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.