The low temperature heat capacity of LaPO 4 and GdPO 4, the thermodynamic functions of the monazite-type LnPO 4 series

Abstract

The heat capacity of lanthanum and gadolinium orthophosphate (LaPO 4 and GdPO 4) with monazite-like structure (i.e., monoclinic) was measured by hybrid adiabatic relaxation calorimetry in the temperature ranges (2 to 380) K and (0.5 to 301) K respectively. A semi-empirical method has been used to describe the total specific heat as the sum of a lattice and an excess component. An excess contribution due to magnetic effects was observed for GdPO 4 below T = 12 K and it was found that GdPO 4 orders magnetically below 0.8 K. Combining the results obtained for both compounds, the excess heat capacity for CePO 4 was deduced and compared to the excess heat capacity derived from the crystal-field energies. A good agreement has been found between the experimental heat capacity and the calculated one. The thermodynamic functions of the monazite-type LnPO 4 series were determined.