Metavalent-Like Bonding Character Drives Giant Anharmonicity and Ultralow Thermal Conductivity in Rocksalt BeO

Identifying materials with intrinsically low lattice thermal conductivity (κL) underpins thermoelectric and phase-change technologies, yet simple structural descriptors often fail to capture how chemical bonding controls lattice dynamics. Here, we compare thermal transport in two BeO polymorphs of identical chemistry and atomic mass: covalent zincblende (zb) and metavalent-like rocksalt (rs). Despite being denser, rs-BeO exhibits a κL more than an order of magnitude below zb-BeO (24 vs 357 W m-1 K-1 at 300 K), breaking the conventional density-κL correlation. Ab initio bonding analysis traces this anomaly to metavalent-like bonding in rs-BeO, which produces electron delocalization, long-range interatomic force constants, and giant quartic anharmonicity that drives strong four-phonon scattering, whereas stiff covalent bonding sustains efficient heat conduction in zb-BeO. Quantitative modeling of the rocksalt phase requires explicit phonon renormalization and four-phonon scattering, effects negligible in zb-BeO. Bonding character, rather than density, governs thermal transport in this ultrawide-bandgap oxide.