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

TLDR

Density‑functional‑theory calculations were used to determine lattice parameters, elastic constants, electronic properties, and charge distributions of ideal A₂B₂O₇ pyrochlores (A = La, Y; B = Ti, Sn, Hf, Zr), with thermal properties inferred from the elastic data and charge analysis revealing hybridization between O 2p and B d states. The study finds that heavier B ions lower sound velocity and Debye temperature, that Ti, Hf, and Zr exhibit anomalously large dynamical charges due to O 2p–B d hybridization, and that ionicity rises in the order Sn < Ti < Hf < Zr.

Abstract

Density Functional Theory calculations have been performed to obtain lattice parameters, elastic constants, and electronic properties of ideal pyrochlores with the composition A$_2$B$_2$O$_7$ (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/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. The 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.