Theoretical study of anisotropic structural, electronic, mechanical and thermodynamic properties of rare-earth (R = Y, La) oxysulfides

Abstract The anisotropic structural, electronic, mechanical and thermodynamic properties of rare-earth (R = Y, La) oxysulfides are calculated by first-principles using density functional theory. From the calculated electronic structure and electron density topology, we find that the crystal structure of R 2 O 2 S is dominated by strong polar R O and R S bonds, and the ionic bond is the main bonding mechanism. The negative formation enthalpies imply that the formation of R 2 O 2 S from reactants (R 2 O 3  + H 2 S) is energetically favorable. The two oxysulfides show obvious anisotropy in elastic properties along different crystallographic directions, where Young's modulus shows stronger anisotropy than bulk modulus. The anisotropy in sound velocity is obtained by solving the Christoffel equation, the sound modes are isotropic at the basal plane ( x - y plane), and weak anisotropy is seen on (1 0 0) crystallographic plane. The temperature dependence of volumetric TECs of both oxysulfides are similar, and the linear TECs in [0 0 1] direction are slightly higher than that of the [1 0 0] direction. At room temperature, the computed average linear TECs for Y 2 O 2 S and La 2 O 2 S are 9.5 × 10 −6  K −1 and 9.4 × 10 −6  K −1 , respectively. Using Slack's model, it is found that thermal conductivities are 10.1 and 5.3 W/m K in a range from 300 K to 1500 K for Y 2 O 2 S and La 2 O 2 S, respectively.