Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 107, Issue 21, Pages 9519-9524Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0912130107
Keywords
first principles computations; lower mantle; thermal properties
Categories
Funding
- National Science Foundation (NSF) [EAR-0530282, EAR-0310139]
- Deparment of Energy (DOE) [DE-FG02-99ER45795]
- Office of Science of the U.S. DOE [DE-AC02-05CH11231]
- EPSRC [EP/F032773/1, EP/F008651/1] Funding Source: UKRI
- Engineering and Physical Sciences Research Council [EP/F032773/1, EP/F008651/1] Funding Source: researchfish
- U.S. Department of Energy (DOE) [DE-FG02-99ER45795] Funding Source: U.S. Department of Energy (DOE)
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Silica (SiO2) 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 Schrodinger's equation. Using ground-state 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 core-mantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense alpha-PbO2 structure above the core-insulating 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.
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