The Effect of Composition on the High-Pressure Behavior of Rare-Earth Phosphate Minerals

A comprehensive study on the effect of composition on the structural and elastic properties of MPO4 (M = Ce3+, Gd3+, Tb3+, Y3+, Sc3+) compounds has been completed. CePO4 and GdPO4 are isostructural with monazite (P21/n), and TbPO4, YPO4, and ScPO4, are isostructural with xenotime (I41/amd). Raman spectra are consistent with previous studies and high-pressure spectra showed no phase transitions up to 10 GPa under hydrostatic conditions. The spectra were used to develop Kieffer-type lattice vibrational models to calculate heat capacities of CePO4 and YPO4 and the results lie within 1-3% of experimental values. Equations of state were calculated from high-pressure single-crystal X-ray diffraction data. Bulk moduli (K0) determined from a 3rd-order Birch-Murnaghan equation of state are: 109(3) GPa for CePO4, 128.1(8) GPa for GdPO4, 141(1) GPa for TbPO4 and 166(1) GPa for ScPO4. The inverse relationship observed between K0 and the ionic radius of the RE3+ is shown to be linear. This equation can be used to predict K0 for other rare-earth phosphates. Comparison of these studies, performed under hydrostatic conditions, with previous studies show that MPO4 structures are sensitive to shear stresses created from non-hydrostatic environments. The first structural study of a monazite, GdPO4, is also reported. Compression mechanisms are comprised of "squishing" the GdO9 polyhedra and inter-polyhedral movement. This study and the axial compressibility data for Ce-, Tb-, Gd- and ScPO4 suggest that the compression mechanisms favored by MPO4 compounds are those which remain rigid parallel to polyhedral chains.