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 REPO ₄ 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 REPO ₄ compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO ₄ 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.