Pressure and temperature dependence of the static dielectric constants and elastic anomalies of ZnF2

Abstract

The static dielectric constants εa and εc of ZnF2 have been studied in the range 4–296 °K at zero pressure and in the pressure range 0–5 kbar at 296, 195, and 78 °K. εa and εc exhibit normal temperature behavior. εc decreases with pressure as is normally the case. εa, on the other hand, exhibits an anomalous increase with pressure at room temperature. This anomalous increase is reduced with decreasing temperature and by 78 °K has completely disappeared. The pressure and temperature results are combined with thermal expansion and compressibility data to evaluate the pure volume and the pure temperature dependence of the dielectric constants. The dielectric constants are described by a Clausius–Mossotti relation. The Szigeti effective charge ratios have been determined for the transverse optical (TA) modes along the [100] and c axes. These ratios are found to be rather small, being 0.44 and 0.54 for the two axes, respectively, suggesting that the bonding is partially covalent. The principal components for the Gruneisen tensor have also been determined. Below 80 °K, they are negative. This is due to the negative mode-Gruneisen parameters of the transverse acoustic (TA) modes associated with the C44 and Cs elastic constants. The possible correlation between the pressure anomaly of εa and temperature anomaly of C44 is discussed and attributed to the instability of the chains of F− octahedra.