High-temperature calorimetry of (La1−xLnx)PO4 solid solutions

The enthalpy increment of the monazite-type solid solutions of LaPO4 with NdPO4, EuPO4 and GdPO4 has been measured by drop calorimetry at T = 1000 K. The results show deviations (excess enthalpy) from ideal behaviour that have been interpreted in terms of lattice strains resulting from the ion size effects of substitution of La3+ by Ln3+. For (La0.5Gd)0.5PO4 also the temperature dependence has been determined for T = (515 to 1565) K, indicating that the excess enthalpy decreases with increasing temperature.