Structure of small oxygen vacancy defects in nonstoichiometric rutile

Abstract

New structural models are derived for oxygen vacancy defects, which occur in impurity and dopant-controlled regimes of slightly substoichiometric rutiles. Two reconstructions of the traditional vacancy model are derived, which offer a natural explanation for high-resolution electron paramagnetic (ESR) observations. Both reconstructions involve simply displacement of one cation from (000) to(−½00), for a vacancy at ( uu0) of the rutile structure. They differ in the distribution of charge-compensating defects associated with the different sites in the immediate vicinity of the defect. Analysis of the electrostatic valencies of the oxygen ions leads to the suggestion that oxygen vacancies in rutile should readily trap H + ions. ESR evidence is reviewed offering confirmation of these predictions. Diffusion mechanisms are also described. These new small defect models offer natural explanations for many conflicting interpretations of physical property measurements of reduced and doped rutiles (e.g., electrical conductivity, thermogravimetry, and anelastic dielectric relaxation). The possibility of directly observing and positively identifying such small defects using high-resolution electron microscopic techniques is considered briefly, as is their relevance to the interpretation of plastic deformation and dynamic strain ageing experiments.