The thermodynamics of CaSiO3 in Earth's lower mantle

TL;DR

This study uses first-principles simulations to analyze the stability, phase diagram, elastic properties, and thermal conductivity of CaSiO3 under Earth's lower mantle conditions, revealing the stable cubic phase and insights into sound velocity and thermal transport.

Contribution

It introduces a comprehensive first-principles approach using SSCHA to accurately model CaSiO3's behavior at extreme conditions, including anharmonic effects and thermal conductivity.

Findings

Cubic CaSiO3 is stable at lower mantle conditions.

The phase boundary between cubic and tetragonal phases is first-order and temperature-dependent.

Thermal conductivity is dominated by particle-like transport, supporting Boltzmann transport models.

Abstract

The lower mantle of Earth, characterized by pressures of 24-127 GPa and temperatures of 1900-2600 K, is still inaccessible to direct observations. In this work, we investigate by first principles the stability, phase diagram, elastic properties, and thermal conductivity of CaSiO3, that constitutes a significant component of Earth's lower mantle. Notably, our simulations capture in full the anharmonic ionic fluctuations arising from the extreme temperatures and pressures of the lower mantle, thanks to the use of stochastic self-consistant harmonic approximation (SSCHA). We show that the cubic phase of CaSiO3 is the stable state at the lower mantle's thermodynamic conditions. The phase boundary between the cubic and tetragonal phases is of first-order and increases linearly from 300 K to 1000 K between 12 GPa and 100 GPa. Accounting for temperature-renormalized phonon dispersions, we evaluate the speed of sound as a function of depth. Our results downplay the role of octahedral rotations on the transverse sound velocity of cubic CaSiO3, advocated in the past to explain discrepancies between theory and experiments. The lattice thermal conductivity, assessed thanks to the recently introduced Wigner formalism, shows a predominance of particle-like transport, thus justifying the use of the standard Boltzmann transport equation even in a system with such strong ionic anharmonicity.