An Ab-initio assessment of thermo-elastic properties of CaCO3 polymorphs: Calcite case

Abstract

Calculation of total pressures, bulk modulus and thermal expansion, heat capacity and entropy of calcite were obtained through evaluation, at the Ab-initio level, of the unit cell volume dependence of the total static energy, and of the frequencies of the vibrational normal modes, at the Γ point, within the framework of the quasi-harmonic approximation. Thermo-elastic properties at zero pressure were corrected for the effects of phonon dispersion by calculating the vibrational frequencies at the Brillouin zone border, by using a supercell approach. Also, a modified Kieffer’s model was also tested to correct for dispersion effects concerning the acoustic modes only. Moreover, a correction to take into account intrinsic anharmonic contributions was performed by correcting all of the calculated frequencies by means of factors derived from literature experimental data.The algorithm allowed for the determination of (i) the equation of state, (ii) the thermal expansion as a function of pressure and temperature, and (iii) the evaluation of some thermodynamic properties (entropy and specific heat), together with their temperature dependences. The obtained results were compared with experimental data.The present work confirms previously experimentally determined parameters of the equation of state and thermodynamic properties of calcite. At room-temperature conditions, the calculated bulk modulus was 75.6 GPa which is to be compared with the corresponding experimental value of 76.9 GPa. The estimated thermal expansion at room temperature is 1.70⋅10−5K−1, which is in excellent agreement with the experimental datum (1.77⋅10−5K−1). The calculated entropy (S) and the constant pressure specific heat (CP) were respectively 91.4 and 81.8 J mol−1 K−1, which are also in close agreement with the experimental data [91.7 and 83.5 J mol−1 K−1, for S and CP respectively].