| In the earth's lower mantle,silica goes through two phase transitions-from stishovite to CaCl2-type silica and then to ?-PbO2-type silica.This study,we cal-culated the elastic constants and thermal properties of stishovite,CaCl2-type silica and ?-PbO2-type silica(seifertite)at simultaneous high pressure and high tempera-ture based on first principle.The calculated thermal expansion and heat capacity of stishovite agree well with previous reports and they can be fitted into several param-eters.Using ab initio results for structural and vibrational properties,the elastic con-stants,seismic velocities of ?-PbO2-type silica were obtained over a wide pressure and temperature range.The phase boundary of these three phases were also obtained in this study.Combined the phase boundary and seismic velocities of these three phases silica,the velocity jump across the CaCl2-type silica and ?-PbO2-type silica of P wave and S wave are +0.5%and-2%to-3%,respectively.The seismic observation of shear velocity decrease in some regions at 2500 km in lower mantle may be caused by this phase transition.Compared to the velocity of PREM model,the P wave velocity of silica is 12%larger and the S wave velocity of silica is 14%larger.Due to the high positive velocity contrast,the high velocity shear wave block in the lower mantle may be caused by the enrichment of silica. |
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