Internal Friction in Rutile Containing Point Defects

Abstract

The type of point defect which is introduced into rutile by reduction caused by heating in vacuum is not firmly established although both oxygen vacancies and titanium interstitials have previously been suggested. Symmetry arguments are used to determine the orientation dependence of internal friction for both. A peak with activation energy of 23.5 kcal/mole with a computed standard error of 1 kcal/mole was observed with stress along the [100] direction which is forbidden for isolated oxygen vacancies with the full symmetry of the oxygen sites; oxygen vacancies of lower symmetry might cause this peak, but would also cause another peak which was not observed. The observed peak is permitted for titanium interstitials. Such interstitials should also cause a peak for stress directed along [110], but no such peak was observed. Full agreement with the present experimental results can be obtained by assuming that titanium interstitials are associated in pairs over the temperature range of the peak. This interpretation may not be the only way of explaining the data, particulary since we have not considered the effect of chemical impurities in association with point defects or more complicated phenomena. The evidence strongly suggests that the predominant point defect in vacuum-reduced rutile is neither a simple oxygen vacancy nor a simple titanium interstitial as is frequently assumed.