HR: 11:20h
AN: V21E-05    [PDF]
TI: {\it Ab initio} studies of phonon softening at high pressure in quartz SiO$_{2}$
AU: * Choudhury, N
EM: n.choudhury@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: * Choudhury, N
EM: n.choudhury@gl.ciw.edu
AF: Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085 India
AU: Chaplot, S L
EM: chaplot@magnum.barc.ernet.in
AF: Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085 India
AB: The silica polymorph quartz exhibits several interesting properties including pressure induced amorphization, high pressure phase transitions, anomalous elastic properties, negative Poisson ratios, soft mode behavior, etc. {\it Ab initio} density functional calculations of quartz are used to understand the changes in phonon frequencies and elastic properties under bulk and uniaxial compression. Non-hydrostatic stresses are believed to play an important role in the pressure induced amorphization. \\ We have undertaken density functional LDA calculations of quartz using extended norm conserving pseudopotentials to compute the phonon frequencies at various points in the Brillouin zone as a function of pressure. The calculated equation of state and phonon frequencies are found to be in good agreement with experimental data. A 466 cm$^{-1}$ Raman active A$_{1}$ mode shifts considerably under uniaxial pressure along the c-axis as compared to bulk compression in agreement with reported Raman data. This mode involving symmetric bending of the Si-O-Si bond is believed to have a principal role in the compression mechanism of quartz. A zone boundary (1/3, 1/3, 0) K-point phonon mode becomes soft at high pressures. The mode softening is related to the high pressure amorphization and phase transitions of quartz and these studies suggest that the mean amorphization pressure can be lowered under non-hydrostatic conditions.
DE: 3900 MINERAL PHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting