HR: 1340h
AN: MR23A-0185    [Abstracts]
TI: Phase stability of CaSiO$_3$ perovskite at high pressure
AU: * Brodholt, J
EM: j.brodholt@ucl.ac.uk
AF: University College London, Dept. of Earth Sciences, Gower Street, London, WC1E 6BT United Kingdom
AU: Caracas, R
EM: r.caracas@gl.ciw.edu
AF: Carnegie Insitution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Wentzcovitch, R M
EM: wentzcov@cems.umn.edu
AF: University of Minnesota, Dept. of Chemical Engineering and Materials Science, 421 Washington Ave., SE, Minneapolis, MN 55455 United States
AU: Price, D G
EM: d.price@ucl.ac.uk
AF: University College London, Dept. of Earth Sciences, Gower Street, London, WC1E 6BT United Kingdom
AB: We investigate by first-principles the structural behavior of CaSiO$_3$ perovskite throughout Earth's lower mantle pressure regime. We confirm that the cubic perovskite modification is unstable at all pressures. The structure is stabilized by SiO$_6$ octahedral rotations corresponding to unstable phonon modes in the R\{1/2 1/2 1/2\} and M\{1/2 1/2 0\} high-symmetry points of the ideal cubic perovskite structure. The freezing-in of these vibrational modes lower the symmetry to tetragonal, orthorhombic, rhombohedral, or cubic. The relative energy differences between all the investigated phases are very small. In general, the tetragonal structures are energetically preferred to the orthorhombic ones. At pressures relevant for the lower mantle, the lowest-in-energy structure has I4/mcm symmetry, followed in increasing order of energy by P$_2$/nmc, Pnma and Imma, P4/mbm, I4/mmm and Im$\overline{3}$, Pm3m, while the highest in energy structure has R$\overline{3}$c symmetry. This trend is preserved at all pressures, with the exception of the P4/mbm phase which is metastable with respect to Pm$\overline{3}$ at low pressures. All the structures have very similar densities, about 0.25 g/cm$^3$ larger than PREM's, with a relatively constant difference at all pressures. CaSiO$_3$ is characterized by bulk modulus higher than PREM bulk modulus by about 75 GPa at 80 GPa pressure and about 125-145 GPa at the base of the D'' layer, at 135 GPa pressure. If present in the lower mantle, CaSiO$_3$ perovskite, due to its larger bulk modulus may become seismically visible.
DE: 3620 Crystal chemistry
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting