The low temperature heat capacity of LaPO4 and GdPO4, the thermodynamic functions of the monazite-type LnPO4 series

The heat capacity of lanthanum and gadolinium orthophosphate (LaPO4 and GdPO4) 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 GdPO4 below T = 12 K and it was found that GdPO4 orders magnetically below 0.8 K. Combining the results obtained for both compounds, the excess heat capacity for CePO4 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 LnPO4 series were determined.