Phase stability and shear softening inCaSiO3perovskite at high pressure

Abstract

We predict the phase diagram of CaSiO 3 perovskite, finding the tetragonal I4/mcm structure transforming to cubic Pm3m with increasing temperature. The transition temperature is 1150 K at 0 GPa, and 2450 K at 140 GPa. The c / a ratio of the tetragonal structure is 1.018 at 100 GPa and increases on compression, as does the static enthalpy difference between tetragonal and cubic structures. The elastic constants of the tetragonal phase at static conditions differ substantially from those of the cubic phase with the Voigt-Reuss-Hill shear modulus 29% less at 100 GPa. Computations are based on density functional theory in the local density and generalized gradient approximations. The phase diagram and high temperature elastic constants are computed using a mean field theory with parameters of the Landau potential determined via structurally constrained density functional theory calculations. We present a simple scheme for systematically searching for the ground state over all perovskite structures derivable from octahedral rotations within the context of symmetrypreserving relaxation, which confirms tetragonal I4/mcm as the ground state in density functional theory. We argue that the experimental x-ray diffraction pattern can be explained by the I4/mcm phase by considering the development of preferred orientation under uniaxial compression.

Small shear modulus of cubic CaSiO3 perovskite

“…A possible ferroelastic behavior associated with a drastic increase in shear modulus across the tetragonal-cubic phase transition of CaPv was also suggested [Stixrude et al, 1996[Stixrude et al, , 2007. In this study, we investigated the elasticity of cubic CaPv through FPMD computations, and we found a smaller (~50% at 100 GPa) shear modulus of cubic CaPv than was determined by Karki and Crain [1998] (Figure 1b).…”

“…On the other hand, Li et al [2006b] used a tetragonal supercell rather than a cubic primitive cell and found that tetragonal CaPv was stable at LM P-T conditions due to insufficient convergence of the electronic structure with the Brillouin zone sampled only on the Γ point as shown in our previous study [see Kawai and Tsuchiya, 2014]. Then, they reported the elasticity of tetragonal CaPv at high temperatures, which is drastically different from the computation at static conditions by Crain, 1998 andStixrude et al [2007]. Accordingly, no theoretical studies have investigated the elasticity of cubic CaPv at LM P-T conditions.…”

“…Averages are unchanged within numerical uncertainty compared with those of longer 10,000 time steps and found to be well converged. Simulations were performed in the P-T range from 0 to 200 GPa and from 1000 to 5000 K with a 2 × 2 × 1 supercell (80 atoms; 80SC) of the tetragonal conventional cell as cell parameters were set to satisfy cubic conditions, namely, a ¼ b ¼ ffiffi ffi 2 p c. Although the cubic unit cell was used in all the previous studies [Karki and Crain, 1998;Karki and Stixrude, 1999;Stixrude et al, 2007], we found that the cubic unit cell cannot take into account the rotational relaxation of SiO 6 octahedra and becomes hard as discussed later in the section 4. Therefore, the 80SC was used in order to consider the rotational relaxation of SiO 6 .…”

“…between the tetragonal and cubic structure was also indicated because of the considerably large shear modulus [Stixrude et al, 2007], which is considered to be a good indicator for the presence of chemically heterogeneous materials such as subducted oceanic crust [e.g., Komabayashi et al, 2007]. On the other hand, Li et al [2006b] used a tetragonal supercell rather than a cubic primitive cell and found that tetragonal CaPv was stable at LM P-T conditions due to insufficient convergence of the electronic structure with the Brillouin zone sampled only on the Γ point as shown in our previous study [see Kawai and Tsuchiya, 2014].…”

Lower mantle hydrogen partitioning between periclase and perovskite: A quantum chemical modelling

“…Therefore, the anharmonic part representing a higher order of approximation is negligible; (ii) in general, apart from some particular cases such as Ca-perovskite (Stixrude et al, 2007), anharmonicity loses impact at high-pressure (Stacey and Isaak, 2003;Wentzcovitch et al, 2010).…”