Pressure-Induced Reversal of Thermal Anisotropy in Bi2O2Se

TL;DR

This study reveals that applying pressure to Bi2O2Se reverses its thermal anisotropy without phase change, due to phonon dispersion changes and lattice compression, informing future thermal management strategies.

Contribution

It uncovers the pressure-induced reversal of thermal anisotropy in Bi2O2Se and explains the underlying phonon and lattice mechanisms involved.

Findings

Thermal anisotropy reverses at 60 GPa without phase transition.

Phonon dispersion changes lead to anisotropy reversal.

Lattice compression affects Bi lone pair activity.

Abstract

Bi2O2Se is an emerging semiconductor with intrinsically low thermal conductivity, making it a promising material for thermoelectric applications. Hydrostatic pressure can effectively tunes the thermal conductivity, with various pressure-dependent trends reported. However, its impact on thermal anisotropy, particularly in the highly anisotropic Bi2O2Se, remains poorly understood. Here, we report a pressure-driven reversal of thermal anisotropy: k_z < k_x at 0 GPa transforms into k_z > k_x at 60 GPa without phase transition. This stems from distinct phonon dispersions along the x- and z-directions under pressure, leading to a reshaped group velocity landscape. Below 10 meV, vz > vx at both pressures, with a much greater advantage at 60 GPa. Above 10 meV, vx > vz at 0 GPa; however, the difference nearly vanishes at 60 GPa. These changes result from anisotropic lattice compression, with the z-axis shrinking more significantly than the x-axis and suppressing the lone pair activity of Bi atoms. This study calls for revisiting the pressure dependence of thermal conductivity anisotropy and provides insights for pressure-driven thermal switching applications.