Metal impurities in an oxide: Ab initio study of electronic and structural properties of Cd in rutile TiO₂

Abstract

In this work we undertake the problem of a transition metal impurity in an oxide. We present an ab initio study of the relaxations introduced in TiO₂ when a Cd impurity substitutionally replaces a Ti atom. Using the full-potential linearized-augmented-plane-wave method, we obtain relaxed structures for different charge states of the impurity and computed the electric-field gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are dependent on the charge state of the impurity. This dependence is very remarkable in the case of the EFG and is explained by analyzing the electronic structure of the studied system. We predict fairly anisotropic relaxations for the nearest oxygen neighbors of the Cd impurity. The experimental confirmation of this prediction and a brief report of these calculations have recently been presented [Phys. Rev. Lett. 89, 55503 (2002)]. Our results for relaxations and EFGs are in clear contradiction with previous studies of this system that assumed isotropic relaxations, and point out that no simple model is viable to describe relaxations and the EFG at Cd in TiO₂ even approximately.