High-temperature behaviour of the elastic moduli of LiF and NaF: Comparison with MgO and CaO

The elastic moduli of single-crystal LiF and NaF have been determined by the ultrasonic pulse superposition technique as a function of temperature from T = 298–650° K. These new data are consistent with low-temperature (T < 298° K) data obtained by other ultrasonic pulse techniques and are superior to previous high-temperature data from resonance experiments. The elastic moduli (c) are represented by quadratic functions in T over the experimental temperature range although the curvature is not in the same sense for all modes. For LiF, NaF, MgO and CaO, evaluation of the temperature derivatives of the elastic moduli at constant volume (V) indicates that the elastic moduli are only weakly dependent on T at constant volume. The fluoride—oxide analogue pair LiFMgO both exhibit high-temperature elastic behaviour at approximately the same absolute temperature. Mitskevich's theory and observed K_S-V systematics imply that (?c/?T)_P should be a function of the nearest neighbour distance for rocksalt fluorides and oxides; this result lends further support to a fluorideoxide modelling scheme based on similar ionic radii.