Pressure-induced anomalous suppressed thermal conductivity in MgX (X = Se, Te): Soft phonon modes and enhanced four-phonon scattering

Understanding the mechanism by which pressure modulates lattice thermal conductivity has been a focus in the study of thermal transport. In this work, based on the first-principles calculations combined with the phonon Boltzmann transport equation, this study reveals an anomalous pressure dependence of the thermal conductivity in MgX (X = Se, Te) with the F-43m phase. After applying pressure, the thermal conductivity of MgSe and MgTe decreases by 63% and 74%, respectively, at room temperature. This anomalous relationship is mainly due to the softening of the transverse acoustic (TA) mode. The softening of the TA mode leads to an increase in the Grüneisen parameter, resulting in larger anharmonicity, which in combination with the enlarged scattering channels leads to an increase in the scattering rate, thus suppressing the thermal transport efficiency. The calculations show that the elastic property analysis is applicable to measure the magnitude of anharmonicity in both materials. In addition, it is found that higher-order phonon scattering is a non-negligible factor in studying the effect of pressure on thermal transport properties. After considering the four-phonon scattering process, the thermal conductivity of MgSe and MgTe decreases by more than 25% under pressure compared to values calculated when only the three-phonon scattering process is considered. These findings reveal the mechanisms behind the anomalous pressure dependence of thermal conductivity and highlight the importance of high-order phonon scattering in the context of pressure-modulated thermal transport.