Bonding Characteristics, Thermal Expansibility, and Compressibility of RXO4 (R = Rare Earths, X = P, As) within Monazite and Zircon Structures

Systematically theoretical research was performed on the monazite- and zircon-structure RXO4 (R = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; X = P, As) series by using the chemical bond theory of dielectric description. The chemical bond properties of R−O and X−O bonds were presented. In the zircon phase, the covalency fractions of X−O bonds increased in the order of V−O < As−O < P−O, which was in accordance with the ionic radii and electronegative trends, and the covalency fractions of R−O bonds varied slightly due to the lanthanide contraction. While in the monazite phase, both R−O and X−O bonds were divided into two groups by their covalency fractions. The contributions from the bond to the lattice energy, linear thermal expansion coefficient (LTEC), and bulk modulus were explored. The X−O bonds with short bond lengths and high chemical valence made greater contributions to the lattice energy and performed nearly rigidly during the deformation. A regular variation of lattice energy, LTEC, and bulk modulus with the ionic radii of the lanthanides was observed in both monazite and zircon phases.