Ab initio calculations of structural and electronic properties of Y2Ti2O7 and Cd2Nb2O7

The full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory was used to investigate the structural and electronic properties of Y2Ti2O7 and Cd2Nb2O7. In this approach, the generalized gradient approximation was used for the exchange–correlation potential. We have firstly optimized the internal parameter and used it to calculate the ground state properties such as lattice constant, bulk modulus and its derivative as well as angles and inter-atomic distances. The results were in good agreement with available experimental measurements. We find that Y2Ti2O7 and Cd2Nb2O7 are highly incompressible and thus can be good candidates for hard materials. Calculations of band structure, density of state and charge density were also performed to describe the orbital mixing and the nature of chemical bonding. Both Y2Ti2O7 and Cd2Nb2O7 exhibit indirect band gaps. The mixture of B-d and O-2p states emphasis the covalent nature of the B–O bond that should explain the hardness of pyrochlore compounds. The small distances between oxygen and A/B atoms also lead to higher covalency of the bonds.