Elastic stiffness coefficients of thenardite and their pressure and temperature dependence

Abstract

AbstractThe elastic stiffness coefficients,cij, of orthorhombic Na2SO4thenardite (space group Fddd) were measäured with an ultrasonic plane wave technique at ambient temperature as a function of hydrostatic pressure in the range of 0.1–70 MPa. The variation of thecijin the range of 1–5000 MPa was studied with density functional theory (DFT) based calculations. The experimental results and the DFT calculations were used to derive a force-field model, which was then employed to compute lattice parameters and elastic stiffness tensors of thenardite and of two other Na2SO4polymorphs as functions of the temperature based on quasi-harmonic lattice dynamics. The structural parameters of the three polymorphs measured at high temperatures are reproduced to within 1.7% by the present calculations. Phases II (space groupPbnm) and III (Cmcm) appear to have significantly higher entropies than thenardite in agreement with their metastable formation at higher temperatures.