First-principles study of the lattice thermal conductivity of the Si-O-H system at high pressure

Using first-principles calculations and the linearized phonon Boltzmann transport equation, we systematically investigate the phonon dispersion, lattice thermal conductivity, group velocities, phonon lifetimes, and Grüneisen parameters of three Si-O-H compounds (SiOH₂, SiO₂H₂, and (SiO₂)₂(H₂O)) at high pressures. Our results reveal remarkably high room-temperature thermal conductivities of 603.65 W m^-1 K^-1 (SiOH₂), 464.62 W m^-1 K^-1 (SiO₂H₂), and 321.22 W m^-1 K^-1 ((SiO₂)₂(H₂O)), which are comparable to those of conventional high-pressure planetary materials like Si and SiO₂ (∼10³ W m^-1 K^-1). These exceptional values mainly arise from synergistic effects of high group velocities, prolonged phonon lifetimes, and low Grüneisen parameters. Besides, we also observe that incorporating H₂O into Si to form SiOH₂ will reduce the thermal conductivity compared to that of pure Si at elevated temperatures due to the combined effects of high-frequency localized vibrations from light hydrogen atoms and strong anharmonic phonon scattering induced by polar Si-O bonds. Such suppression of thermal conductivity in Si-O-H systems may alter the conductivity distributions in the convective layer and impair the heat transport efficiency, which may potentially explain the anomalously low heat flux inside Uranus and Neptune. Our work elucidates the physical mechanism of ultrahigh-thermal conductivity in the Si-O-H compounds under extreme conditions and offers critical insights into understanding the thermal evolution of ice giants.