Comparative ab initio study of electronic, optical and chemical bonding properties of pyrochlores, Y₂B₂O₇ (B=Ti ⁴, Sn⁴)

The nature of chemical bonding that is responsible for the disparity between the optical properties of the impurity ions in the pyrochlores, Y ₂Sn₂O₇ and Y₂Ti₂O ₇ has been quantified by computing in the density functional theory framework the structural, physical electronic and optical properties of the host lattices. The partial density of states calculations and the estimation of the Mulliken charges indicate the significant different behavior of the oxygen ions in the two different crystallographic positions (48f and 8b) of the pyrochlore lattice. The hybridization between the filled 2p orbital of oxygen ion on the 48f site (O48f2-) and the empty 3d⁰ orbital of Ti⁴ results in an anomalously lower charge on O48f2- in Y₂Ti₂O ₇ when compared with that in Y₂Sn₂O₇ with filled Sn⁴ 4d¹⁰ orbital. This covalent mixing is chiefly responsible for the disparity between the optical properties of the impurity ions in Y₂Sn₂O₇ and Y ₂Ti₂O₇. Additional information on the nature of chemical bonding is provided by calculating the influence of external hydrostatic pressure on the interatomic distances and the elastic properties of the pyrochlores.