First principles calculations of structural, elastic and electronic properties of XO2 (X = Zr, Hf and Th) in fluorite phase

First principles density functional theory calculations are carried out to investigate the structural, elastic and electronic properties of XO2 (X = Zr, Hf and Th) in the fluorite structure. We use the full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient approximation (GGA). The ground state properties such as lattice parameter, elastic constants, inter-atomic distances and angles, bulk modulus and its pressure derivative were calculated; the results are compared with previous calculations and experimental data when available. We find that HfO2 has the highest strength corresponding to the higher hardness, compared with ZrO2 and ThO2. The electronic structure calculations show that these materials are weakly ionic and exhibit wide band gaps as well as flat valence bands, which are typical of insulator. We conclude that the fluorite structure becomes less strong and the band gap increases when X is a larger ion; the ionic character of O–O bond also tends to be more significant while the chemical nature of X–O bond is not affected by the size effect.