High-pressure thermoelastic and structural properties of KCaF3 perovskite in the low temperature Pbnm phase

Abstract

The thermoelastic and high pressure crystallographic properties of KCaF3 perovskite have been investigated by neutron powder diffraction techniques to a maximum pressure of 6.7 GPa. The effect of increasing pressure is to increase the structural distortion away from the ambient temperature and pressure crystal structure, whilst retaining the space group Pbnm to the highest pressure measured. Both primary order parameters increase with increasing pressure, with no evidence for the proximity of a perovskite - perovskite phase transition at slightly higher pressures. Using supplementary low temperature neutron diffraction data, and the published isobaric heat capacity, the thermoelastic properties of KCaF3 are well described by a two-term Debye model with characteristic temperatures 180 K, and 482 K, and mean Grüneisen constant of 1.5. The isothermal bulk modulus and the individual polyhedral bulk moduli have been determined from Birch-Murnaghan third, and second order equations-of-state, respectively. The measured bulk modulus, 48.2 GPa, is consistent with the approximate linear trend with inverse molar volume that has been observed in other AIBIIF3 perovskite-structured compounds.