P-V-T equation of state and thermoelastic properties of shortite Na2Ca2(CO3)(2) from first principles

Theoretical study of structural, thermodynamic and dynamic properties of the double Na-Ca carbonate, shortite, was carried out with density functional theory. Local density approximation (LDA) and generalized gradient approximation in the Perdew-Burke-Ernzerh of (PBE) parameterization were used. Calculated structural and elastic properties exhibit typical behavior for both employed methods. At the same time, equation of state P(V) and bulk modulus, obtained by means of LDA and PBE are significantly different from each other at low pressure and high temperature. This difference is caused by dynamic instability of crystal which boundary calculated using LDA always lies at negative pressure region, while in the case of PBE calculations, this limit manifests itself at positive pressure resulting to features in equation of state P(V) and the bulk modulus behavior. Results indicate that accuracy of used theoretical method is critical for understanding the cause of shortite lattice instability at low pressure and sufficiently high temperature.