Anisotropy in elasticity and thermal conductivity of monazite-type REPO4 (RE = La, Ce, Nd, Sm, Eu and Gd) from first-principles calculations

Starting from theoretical calculations based on LSDA, the authors compute the lattice parameters, cohesive energies and formation enthalpies of monazite-type REPO4 compounds. The calculated values are satisfactory compared with the experimental results from the elastic constants obtained, the mechanical moduli are evaluated using the strain–stress method. The predicted bulk, Young’s and shear moduli are in good agreement with the experiments. It is shown that the mechanical moduli are low (<200 GPa) and also increase from LaPO4 to GdPO4. The three-dimensional contours and their planar projections of Young’s modulus are plotted to illustrate the anisotropy in elasticity. It is found that Young’s moduli of all monazite-type REPO4 show strong dependence on direction. The linear thermal expansion coefficients are calculated using the empirical method, and the values are in the range 9 × 10−6–12 × 10−6 K−1. Using Clarke’s and Slack’s models, the thermal conductivities of REPO4 compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO4 are also explained based on the observed monazite to zircon-type transformation in experiment. Solving the Christoffel equation for monoclinic symmetry, the anisotropy in thermal conductivity is investigated. The results indicate that the total lattice thermal conductivities of monazite-type REPO4 show weak dependence on direction. Meanwhile, their sound velocities exhibit strong anisotropic properties.