High-P, T phase relations in the NaAlSi2O6 system from first principles computation
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Vibrational density of states of the NaAlSi2O6 jadeite and NaAlSiO4 calcium ferrite (CF)-type, and SiO2 stishovite is calculated as a function of pressure up to 50 GPa using density functional perturbation theory. The calculated frequencies are used to determine the thermal contribution to the Helmholtz free energy within the quasi-harmonic approximation and to derive the equation of state and several thermodynamic properties of interest. A dissociation of jadeite into a mixture of a CF-type phase and stishovite is predicted to occur at 23.4 GPa and 1,800 K with a positive Clapeyron slope of 2.8 MPa/K. Elastic anisotropy for jadeite, the CF-type phase, and stishovite also computed clearly shows that stishovite and the CF-type phase are the most anisotropic and isotropic in these three phases, respectively.
KeywordsThermodynamic property Vibrational density of states Phase transition Jadeite CF phase First principles
We thank Shinji Yamamoto for valuable discussions. This work was completed under the supports in part of KAKENHI (Grant No. 23540560) to TT and of Ehime Univ. Global Centers of Excellence program ``Deep Earth Mineralogy’’ to TT.
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