Emergence of small defect contrast within HREM images of nonstoichiometric rutile

Abstract

Calculations of HREM images have been used to define the conditions for which interpretable information may be obtained concerning the nature of small defects (traditionally called “point defects”) and their aggregation to form extended defects in nonstoichiometric rutile. At specific crystal thickness and lens defocus conditions as few as three small defects (aligned along the projection axis) become visible in lattice images. The emergence of stronger contrast, as extended defects develop, should allow the interstitial or vacancy nature of the defects to be distinguished. Experimental HREM images are also presented which show a variety of “small defect contrasts” for [001], [010] and [111] projections of nonstoichiometric rutiles (TiO 2- x ; 0.006 ≦ x ≦ 0.02). None of these allow interstitial versus vacancy defects to be distinguished directly, although some show evidence for aggregates of less than 10 small defects.