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

摘要

Density Functional Theory calculations have been performed to obtain latticeparameters, elastic constants, and electronic properties of ideal pyrochloreswith the composition A$_2$B$_2$O$_7$ (where A=La,Y and B=Ti,Sn,Hf, Zr). Somethermal properties are also inferred from the elastic properties. A decrease ofthe sound velocity (and thus, of the Debye temperature) with the atomic mass ofthe B ion is observed. Static and dynamical atomic charges are obtained toquantify the degree of covalency/ionicity. A large anomalous contribution tothe dynamical charge is observed for Hf, Zr, and specially for Ti. It isattributed to the hybridization between occupied $2p$ states of oxygen andunoccupied d states of the B cation. The analysis based on Mulliken populationand deformation charge integrated in the Voronoi polyhedra indicates that theionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The chargedeformation contour plots support this assignment.