Anomalous suppression of thermal conductivity in CuInSe2 under hydrostatic pressure: The role of phonon softening and enhanced four-phonon scattering

While pressure-induced modulation of thermal transport is often sought to avoid the introduction of extrinsic defects, the underlying physics of such regulation remains complex. This paper presents a systematic investigation into the pressure-dependent thermal conductivity of CuInSe 2 , utilizing first-principles calculations and phonon Boltzmann transport theory. We report an anomalous 72.7% decrease in lattice thermal conductivity when the material is subjected to 7 GPa of hydrostatic pressure. The origin of this phenomenon lies in the fundamental softening of transverse acoustic modes, a process that intensifies material anharmonicity via an increased Grüneisen parameter and expands the four-phonon scattering phase space. This interplay significantly elevates the phonon scattering rate, thereby impeding heat carrier propagation. At the electronic level, the crystal orbital Hamiltonian analysis provides a structural justification for this softening, as the anti-bonding states of the Cu Se bonds are observed to migrate toward lower energies under compression, resulting in diminished bond stiffness. Additionally, our study underscores the indispensable role of higher-order scattering. The inclusion of four-phonon scattering is found to decrease the predicted thermal conductivity by over 25% relative to the three-phonon approximation at equivalent pressures. These insights reveal that the anomalous thermal suppression in CuInSe 2 is a direct consequence of the synergy between pressure-induced phonon softening and enhanced high-order scattering.