Experimental determination of the high temperature heat capacity of a natural xenotime-(Y) solid solution and synthetic DyPO4 and ErPO4 endmembers

The heat capacity of natural xenotime-(Y) and synthetic DyPO4 and ErPO4 crystals was determined by differential scanning calorimetry (DSC) at temperatures of 298.15 K to 868.15 K and a pressure of 0.1 MPa. The aim of the study was to develop a method to accurately measure the isobaric heat capacity (CP) of rare earth element (REE) phosphates, compare the results to data from adiabatic calorimetric experiments, and evaluate the deviation from ideality of the CP of the natural xenotime-(Y) solid solution. The measured CP data (in J mol−1 K−1) can be described by the relationships: 185.5 − 751.9T−0.5 − 3.261e + 06 T−2 for DyPO4; 207.2 − 1661T−0.5 − 5.289e + 05 T−2 for ErPO4; and 208 − 1241T−0.5 − 2.493e + 06 T−2 for xenotime-(Y); where T is the temperature in K. The heat capacity data for natural xenotime-(Y) were used to determine the excess function for the solid solution, which yields an excess heat capacity ranging between 7.9 and 10.7%, well within the range of the DSC method used in this study. The experiments indicate that xenotime-(Y) forms a non-ideal solid solution. Future DSC studies will provide important data for developing a solid solution model for the incorporation of REE in xenotime-(Y).