The High-Pressure Behavior of Rare-Earth Phosphates

Abstract

This study presents newly-measured isothermal equations of state (EoS) of five rare-earth (RE) phosphates with the xenotime-structure, xenotime-(Sc), xenotime-(Y), and xenotime-(Tb), and two with the monazite-structure, monazite-(Ce) and monazite-(La) determined from high-pressure single-crystal X-ray diffraction studies. The isothermal bulk moduli (K0T) of each rare-earth phosphate, obtained by fitting a third-order Birch Murnaghan EoS to P-V data collected between 1 bar and 10 GPa, vary linearly with the ionic radius (IR) of RE as described by the equation K0T (GPa) = −169.08(29.32) · IR(Å) + 317.21(31.67) (R2 = 0.98). These results resolve scatter among previously-reported bulk moduli of rare-earth phosphates. In addition, a high-pressure structural study of xenotime-(Y) was conducted up to 9.06 GPa under hydrostatic conditions and compared with monazite-(Gd). These studies show that the REOx polyhedra in both compounds control the compressibility of the rare-earth phosphates, although the mechanisms of polyhedral compression differ. This work therefore constrains key structure-property relationships for rare-earth phosphate minerals and indicates that their thermoelastic properties calculated from high-pressure and high-temperature data will be sensitive to compositional impurity and conditions imparted by quasi- or non-hydrostatic stress states in high-pressure experiments.