Pressure-induced anisotropic reversal of thermal conductivity and its implication beyond conventional higher-order phonon scattering in non–van der Waals layered Bi2O2Se

High pressure, exemplifying extreme environments, offers opportunities to extensively probe the thermophysical properties and thermochemical reactions of materials, enabling numerous practical applications. Recent studies highlight nonencapsulated ultrathin semiconducting bismuth oxyselenide (Bi2O2Se) as an exceptional candidate for exploring thermal transport properties under elevated pressure, with potential implication for advanced high-speed and lower-power electronic devices. Here we employ in situ thermal measurements alongside first-principles calculations to uncover a unique switch of thermal conductivity (κL) anisotropy in Bi2 O2 Se, occurring independent of any phase transition phenomenon under pressure. This unique phenomenon, previously unobserved experimentally and unreported theoretically for other known materials, arises from the interplay between higher-order four-phonon processes and interatomic interactions, which are modulated by pressure. These competitive mechanisms alter phonon propagation, leading to a reversal in the anisotropy of κL. Especially, along the cross-plane direction, the pressure-dependent contributions of high-order phonon anharmonicity initially increase but subsequently decrease. Using Raman spectroscopy, a diamond anvil cell, and time-domain thermoreflectance, we systematically probe the intrinsic phonon line shape of phonon branches, as well as κL up to 24 GPa. Our findings reveal the higher-order phonon anharmonicity contribution up to a ∼63% to heat transfer in cross-plane direction, while the in-plane direction effectively bypasses the heating-carrying phonons. This study delivers fundamental insights into understanding novel heat conduction phenomena for the advancement of flexible thermal-energy-related applications under extreme conditions, including pressure sensors designed for wearable technology utilized in deep-sea exploration, oil drilling, and space missions.