Ab initio lattice thermal conductivity of MgSiO₃ across the perovskite-postperovskite phase transition

Lattice thermal conductivity (κ) of MgSiO₃ postperovskite (MgPPv) under the Earth’s lower mantle (LM) high pressure-temperature conditions is studied using the phonon quasiparticle approach by combing ab initio molecular dynamics and lattice dynamics simulations. Phonon lifetimes are extracted from the phonon quasiparticle calculations, and the phonon group velocities are computed from the anharmonic phonon dispersions, which in principle capture full anharmonicity. κ is calculated by using the linearized Boltzmann transport equation. Systematic results of temperature and pressure dependences of κ of both MgPPv and MgSiO₃ perovskite (MgPv) are demonstrated. MgPPv’s and MgPv’s κ are then modeled along the typical geotherm. It is found that throughout the lowermost mantle, including the D” region, κ of MgPPv is ~25% larger than that of MgPv, mainly due to MgPPv’s higher phonon velocities. Such a difference in phonon velocities between the two phases originates in the MgPPv’s relatively smaller primitive cell. Our calculations also suggest that the MgPv to MgPPv phase transition causes a ~20% enhancement in κ of the pyrolitic LM aggregate at the core-mantle boundary.