Studies of cation dopant sites in metal oxides by EXAFS and computer-simulation techniques

Abstract

A combination of extended X-ray absorption fine structure (EXAFS) experiments and computer simulations has been used to probe the environments of cation dopants in three oxide systems of technological importance, viz. lithium niobate (a material with applications in non-linear optics), tin oxide (a component in some types of gas sensor) and rare-earth-dopod β″-alumina (crystals that exhibit novel optical properties). In Fe³⁺-doped LiNbO₃ this work clearly locates the Fe³⁺ ions predominantly on the Li⁺ sites rather than on the Nb⁵⁺ sites and helps resolve a long-standing problem concerning the nature of the dopant environment. The preferred mode of solution of Ga³⁺ ions in SnO₂ is found to be substitution for the host cations with O^2– ion vacancies as the charge-compensating defects. At the concentration of dopant studied (a few mol%) the association of impurity ions and these defects, with the formation of clusters, is shown to be favoured. At this stage of the work the precise location of rare-earth dopants in β″-alumina is difficult; however, the present investigation indicates that a combination of molecular-dynamics simulations and EXAFS should prove a viable approach to this problem.