Electron Counting and High-Pressure Phase Transformations in Metal Hexaborides

Abstract

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.

Structures of LaH10, EuH9, and UH8 superhydrides rationalized by electron counting and Jahn-Teller distortions in a covalent cluster model

“…39,40 Solids containing cluster anions are particularly amenable to electron counting analysis -for example, the conductivities of metal hexaborides, MB 6 , can be predicted from the metal valence state by application of the Wade-Mingos rules. [41][42][43][44] In this study we focus on covalent sub-units in the H-H networks of LaH 10 , 11 EuH 9 , 37 and UH 8 , 45,46 all shown in Figure 1, in order to explain structural trends. LaH 10 is well-studied by theory and experiments, as described above.…”

Structures of LaH10, EuH9, and UH8 superhydrides rationalized by electron counting and Jahn–Teller distortions in a covalent cluster model

“…39,40 Solids containing cluster anions are particularly amenable to electron counting analysis – for example, the conductivities of metal hexaborides, MB 6 , can be predicted from the metal valence state by application of the Wade–Mingos rules. 41–44…”

“…39,40 Solids containing cluster anions are particularly amenable to electron counting analysisfor example, the conductivities of metal hexaborides, MB 6 , can be predicted from the metal valence state by application of the Wade-Mingos rules. [41][42][43][44] In this study we focus on covalently bonded hydrogen clusters in LaH 10 , 11 EuH 9 , 37 and UH 8 , 45,46 all shown in Fig. 1, in order to explain structural trends.…”

Electronic and Structural Properties of MPtxB6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

“…In general, the hexaborides of the CaB 6 type are refractory compounds with high melting temperatures (above 2600 °C for YB 6 ); they are characterized by excellent acid and oxidation resistance at high temperature and high hardness and high bulk moduli. In recent years, a large number of experimental and theoretical research focused on studying the properties of hexaborides under pressure, revealing the rich chemistry of B in the high pressure polymorphs of hexaborides. ,− Several supercells with reduced symmetry for YB 6 have been claimed to exist at ambient conditions [both from experimental studies (Raman spectroscopy) and DFT calculations] and have been suggested to be more stable than the commonly accepted CaB 6 -type structure. − …”