First-principles anharmonic vibrational study of the structure of calcium silicate perovskite under lower mantle conditions

Abstract

Calcium silicate perovskite (CaSiO3) is one of the major mineral components of the lower mantle, but has been the subject of relatively little work compared to the more abundant Mg-based materials. One of the major problems related to CaSiO3 that is still the subject of research is its crystal structure under lower mantle conditions -a cubic Pm3m structure is accepted in general, but some have suggested that lower-symmetry structures may be relevant. In this work, we use a fully firstprinciples vibrational self-consistent field (VSCF) method to perform high accuracy anharmonic vibrational calculations on several candidate structures at a variety of points along the geotherm near the base of the lower mantle, in order to investigate the stability of the cubic structure and related distorted structures. Our results show that the cubic structure is the most stable throughout the lower mantle, and that this result is robust against the effects of thermal expansion.

Deep‐Learning‐Based Prediction of the Tetragonal → Cubic Transition in Davemaoite

“…With increasing temperature, CaPv undergoes a tetragonal (I4/mcm) to cubic (Pm3m) phase transition. Recent works have confirmed that cubic CaPv is stabilized by its strong anharmonic fluctuations at high temperatures (Prentice et al, 2019;Sun et al, 2014;Zhang & Wentzcovitch, 2021). Because tetragonal and cubic CaPv have rather different sound velocities (Gréaux et al, 2019;Thomson et al, 2019), their stability field under mantle conditions can be crucial to understanding velocity heterogeneities in the mantle if this transition occurs at mantle conditions.…”

“…Thermodynamic and thermoelastic properties of CaPv phases are crucial for a better understanding of Earth's interior, particularly for understanding potential compositional heterogeneities in the mantle and the D'' region where basaltic compositions are expected to be abundant (Gréaux et al., 2019; Hirose et al., 2005; Kesson et al., 1994; Wang et al., 2020). Several experiments and theoretical studies have been carried out to unravel CaPv's structure and thermodynamic properties (Caracas et al., 2005; Komabayashi et al., 2007; Li et al., 2006; Prentice et al., 2019; Shim et al., 2002; Stixrude et al., 1996; Sun et al., 2014; Uchida et al., 2009; Zhang & Wentzcovitch, 2021). With increasing temperature, CaPv undergoes a tetragonal ( I 4/ mcm ) to cubic ( Pm $$\overline{3}$$ m ) phase transition.…”

Davemaoite as the mantle mineral with the highest melting temperature

“…At elevated temperature, it is stabilized in a cubic structure with space group Pm3¯m ( 18 – 20 ). Because the kinetic barrier of back conversion is small, the cubic perovskite is easily transformed to low-pressure polymorphs after pressure release; therefore, it is hardly seen at Earth’s surface ( 21 , 22 ). Recently, a rare sample of this phase was found in diamond inclusion and named davemaoite in honor of high-pressure geoscientist D. (Ho-kwang) Mao ( 23 ).…”