Expansivity and compressibility of strontium fluorapatite and barium fluorapatite determined by in situ X-ray diffraction at high-T/P conditions: Significance of the M-site cations

Strontium fluorapatite (SrF-Ap) and barium fluorapatite (BaF-Ap) were synthesized by using a piston-cylinder apparatus at 1. 0 GPa and 800 °C. Their thermal expansivity and compressibility were subsequently investigated by using in situ high-T powder X-ray diffraction method (up to 1,000 °C at ambient P) and by using a diamond-anvil cell coupled with synchrotron X-ray radiation (up to about 5 GPa at room T), respectively. The derived thermal expansion coefficients and bulk moduli were combined with the literature data for calcium fluorapatite (CaF-Ap) and lead fluorapatite (PbF-Ap) to constrain the influence of the M-site cations on the elasticity of the fluorapatites (MF-Ap). The results show that both the size of the M cations and their polarizability affect the thermal expansion coefficients of the MF-Ap. For the axial and volumetric bulk moduli, the size of the M cations plays a dominant role, with some crystallographic features of the MF-Ap being potentially important as well.