HR: 17:15h
AN: DI44A-06    [Abstracts]
TI: High-Accuracy Pressure Scale Computed With Quantum Monte Carlo
AU: * Esler, K P
EM: kesler@ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washingon, DC 20015,
AU: Militzer, B
EM: militzer@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washingon, DC 20015,
AU: Cohen, R E
EM: rcohen@ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washingon, DC 20015,
AB: There has recently been considerable interest in improving pressure calibration standards for ultrahigh pressure studies in the diamond anvil cell. Developing such a standard requires reliable equation of state (EOS) data, however there are at present no primary pressure standards for the megabar range. Recently, cubic boron nitride (cBN) has been suggested as an accurate pressure gauge [1]. Unlike ruby, its structure is highly constrained by symmetry and stable beyond 100 GPa, and it has a well-separated Raman spectrum with sufficient pressure dependence to allow accurate pressure calibration. Calculation of the cBN EOS with density functional theory (DFT) gives reasonable agreement with experiment, but the results from different approximate functionals disagree. Quantum Monte Carlo is a first principles simulation method which circumvents approximate functionals and is widely regarded to provide the most accurate predictions of the properties of solids available. We utilize state-of-the-art QMC methods to obtain the static EOS for cBN. To this, we apply corrections for phonon contributions computed within DFT. The resulting theoretical EOS agrees well with experiment at pressures below 50 GPa, with a deviation from the zero-pressure lattice constant of only +0.25%. Above 50 GPa, our EOS begins to differ from recent measurements by Datchi et al. [2], with a maximum deviation of approximately -10 GPa at 160 GPa, the highest compression achieved experimentally. Possible reasons for this discrepancy, both theoretical and experimental, will be discussed.
[1] Phys. Rev. B 72, 100104R (2005)
[2] Phys. Rev. B 75, 214104 (2007)
DE: 0500 COMPUTATIONAL GEOPHYSICS (3200, 3252, 7833)
DE: 3919 Equations of state
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting