The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2

TL;DR

This study uses ab-initio methods to analyze how a cadmium impurity affects the electronic structure and local relaxations in TiO2, revealing charge-dependent anisotropic relaxations and electric-field gradients.

Contribution

It provides the first detailed ab-initio analysis of charge state-dependent relaxations and electric-field gradients of Cd impurities in TiO2, challenging previous isotropic assumptions.

Findings

EFGs depend on impurity charge state

Relaxations are anisotropic for oxygen neighbors

Contradicts previous isotropic relaxation models

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 replaces substitutionally 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 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 [P.R.L. 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.