Atomistic simulation of SrF2 polymorphs

Electron gas interionic potentials (EGIP) have been developed to determine the equation of state (EOS) of the cubic (C1) and orthorhombic (C23) polymorphs of SrF₂, including the thermal effects by means of a quasiharmonic Debye model. The zero pressure cell parameter (a₀), lattice energy (E_latt), and bulk modulus (B₀) of the C1 phase are computed with errors smaller than 1.2%, 1.2%, and 7.1%, respectively. The predicted EOS is in good agreement with the observed data and satisfies the universal Vinet EOS. For the C23 phase, the optimized zero-p cell parameters a, b, and c and the six fractional coordinates are reported and the pressure dependence of a/a₀, b/b₀, and c/c₀ explored by fitting independent modified Vinet EOS's to the computed data. The analysis reveals a greater compressibility of the C23 phase along the b and c axes than along the a direction. Our calculation predicts the C1⇌C23 equilibrium to occur at p_tr=3.92 GPa, which is between the observed values for the C1-->C23 (p_tr=5.0 GPa) and C23-->C1 (p_tr=1.7 GPa) phase transitions.