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Thermodynamic Properties of Stishovite and CaCl₂-Type Phases of SiO₂, under Earth’s Pressure and Temperature Variation from First-Principles Calculation
Salah Tlili, Khalissa Layadi and Zohir Radi

In the present work, the Debye temperature of the stishovite and CaCl₂-type polymorphs of silica, is calculated in the 0-80 GPa pressure domain of the earth’s mantle condition, based on their structural and elastic properties from previous studies. In addition, their thermodynamic properties of the internal energy, heat capacity, entropy, and free energy are calculated in the 300–3500 K temperature domain for selected pressure values less than and higher than the phase transition pressure at 40 GPa. These computations aim to determine the relationship with the main seismic discontinuities in the 0-80 GPa pressure domain of the Earth’s interior. Our obtained results show that the thermodynamic properties are mainly controlled by the temperature effect rather than the pressure. Also, the obtained results show that the Debye temperature increases with the pressure, where the CaCl₂-type has the highest values of more than 1350 K. Also, at standard temperature and pressure, our obtained results of the internal energy, heat capacity, entropy, and free energy are 3.92 kJoule/mole, 36.65 kJoule/(mole.K), 25.25 kJoule/(mole.K), and -1.68 kJoule/mole, respectively. In the geophysical implication, the obtained results imply a probable link with the seismic discontinuity at 660 km between the upper and lower mantle.

Keywords: pressure; temperature, Debye-temperature, thermodynamic properties; stishovite; CaCl₂-type; SiO₂