Metavalent Bonding-Induced Phonon Hardening and Giant Anharmonicity in BeO

TL;DR

This study uses first-principles calculations to reveal how metavalent bonding in BeO drastically reduces thermal conductivity through enhanced anharmonicity and multi-phonon scattering, providing insights for thermoelectric material design.

Contribution

It demonstrates the impact of metavalent bonding on phonon transport in BeO and introduces key indicators for discovering similar low-thermal-conductivity materials.

Findings

rs-BeO has ultralow thermal conductivity of 24 W/mK at 300K.

Modeling requires temperature-dependent phonon renormalization and four-phonon scattering.

Key indicators include NaCl structure, high Gr"uneisen parameters, and breakdown of Lyddane-Sachs-Teller relation.

Abstract

The search for materials with intrinsically low thermal conductivity ($\kappa_L$) is critical for energy applications, yet conventional descriptors often fail to capture the complex interplay between bonding and lattice dynamics. Here, first-principles calculations are used to contrast the thermal transport in covalent zincblende (zb) and metavalent rocksalt (rs) BeO. We find that the metavalent bonding in rs-BeO enhances lattice anharmonicity, activating multi-phonon scattering channels and suppressing phonon transport. This results in an ultralow $\kappa_L$ of 24 W m$^{-1}$ K$^{-1}$ at 300 K, starkly contrasting with the zb phase (357 W m$^{-1}$ K$^{-1}$). Accurately modeling such strongly anharmonic systems requires explicit inclusion of temperature-dependent phonon renormalization and four-phonon scattering. These contributions, negligible in zb-BeO, are essential for high-precision calculations of the severely suppressed $\kappa_L$ in rs-BeO. Finally, we identify three key indicators to guide the discovery of metavalently bonded, incipient-metallic materials: (i) an NaCl-type crystal structure, (ii) large Gr\"uneisen parameters ($\textgreater$2), and (iii) a breakdown of the Lyddane-Sachs-Teller relation. These findings provide microscopic insight into thermal transport suppression by metavalent bonding and offer a predictive framework for identifying promising thermoelectrics and phase-change materials.