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

摘要

In this work we undertake the problem of a transition metal impurity in anoxide. We present an ab-initio study of the relaxations introduced in TiO2 whena Cd impurity replaces substitutionally a Ti atom. Using the Full-PotentialLinearized-Augmented-Plane-Wave method we obtain relaxed structures fordifferent charge states of the impurity and computed the electric-fieldgradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, aredependent on the charge state of the impurity. This dependence is veryremarkable in the case of the EFG and is explained analyzing the electronicstructure of the studied system. We predict fairly anisotropic relaxations forthe nearest oxygen neighbors of the Cd impurity. The experimental confirmationof this prediction and a brief report of these calculations have recently beenpresented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs arein clear contradiction with previous studies of this system that assumedisotropic relaxations and point out that no simple model is viable to describerelaxations and the EFG at Cd in TiO2 even approximately.