First-principles study of structural, elastic, and bonding properties of pyrochlores

Density functional theory calculations have been performed to obtain lattice parameters, elastic constants, and electronic properties of ideal pyrochlores with the composition A₂B₂O₇ (where A=La,Y and B=Ti,Sn,Hf,Zr). Some thermal properties are also inferred from the elastic properties. A decrease of the sound velocity (and thus, of the Debye temperature) with the atomic mass of the B ion is observed. Static and dynamical atomic charges are obtained to quantify the degree of covalency or ionicity. A large anomalous contribution to the dynamical charge is observed for Hf, Zr, and specially for Ti. It is attributed to the hybridization between occupied $2p$ states of oxygen and unoccupied d states of the B cation. An analysis based on Mulliken population and deformation charge integrated in the Voronoi polyhedra indicates that the ionicity of these pyrochlores increases in the order Sn-Ti-Hf-Zr. The charge deformation contour plots support this assignment.