Prediction of structural, electronic and elastic properties of Y2Ti2O7 and Y2TiO5

Density functional theory calculations have been performed to study the structural, electronic and elastic properties, and formation energies, of pyrochlore Y2Ti2O7 and orthorhombic Y2TiO5. These oxides are among the hardening features in nano-dispersion-strengthened ferritic alloys that are promising for future advanced fission/fusion energy applications. Calculations employ pseudo-potentials generated within the projector-augmented wave (PAW) method and the generalized gradient approximation (GGA) as well as the local density approximation (LDA) to the exchange-correlation functional. The calculated equilibrium structural parameters agree well with experiment for both oxides. The PAW-GGA yields the better bulk modulus of Y2Ti2O7, and hence is used for all the elastic calculations. A complete set of elastic parameters for both oxides are reported. Analysis of electron localization functions, charge densities, and densities of states suggest that the oxide bonds are highly ionic, but with some covalent character. In both cases the Ti-O bonds exhibit higher covalency than the Y-O bonds.