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The elastic stiffness coefficients,  c   ij  , of orthorhombic Na 2 SO 4  thenardite (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 the  c   ij   in 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 Na 2 SO 4  polymorphs 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 group  Pbnm ) and III ( Cmcm ) appear to have significantly higher entropies than thenardite in agreement with their metastable formation at higher temperatures.

Elastic stiffness coefficients of thenardite and their pressure and temperature dependence