Quantum Monte Carlo computations of phase stability, equations of state, and elasticity of highpressure silica
Abstract
Silica (SiO_{2}) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and understanding pure silica is a prerequisite to understanding the rocky part of the Earth. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrödinger's equation. Using groundstate QMC plus phonons within the quasiharmonic approximation of density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth's coremantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense αPbO_{2} structure above the coreinsulating D" layer, but the absence of a seismic signature suggests the transition does not contribute significantly to global seismic discontinuities in the lower mantle. However, the transition could still provide seismic signals from deeply subducted oceanic crust. We also find an accurate shear elastic constant for stishovite and its geophysically important softening with pressure.
 Publication:

Proceedings of the National Academy of Science
 Pub Date:
 May 2010
 DOI:
 10.1073/pnas.0912130107
 arXiv:
 arXiv:1001.2066
 Bibcode:
 2010PNAS..107.9519D
 Keywords:

 Condensed Matter  Materials Science
 EPrint:
 20 pages, 4 figures, 1 table