TiK-edge XANES studies of Ti coordination and disorder in oxide compounds: Comparison between theory and experiment

Experimental Ti K-edge x-ray-absorption near-edge structure (XANES) spectra for a variety of Ti(IV)-bearing crystalline oxide model compounds are compared with those calculated using the ab initio multiple-scattering code FEFF7. A scattering-theoretic interpretation of various features in the experimental spectra, including pre-edge and main-edge peaks, is presented together with an interpretation of the effects of disorder. The observed pre-edge features are found to vary in both position (by ≈2±0.1eV) and normalized height (from ≈0.04 to 1.0±0.05) as a function of Ti coordination (4, 5, or 6 oxygen nearest neighbors), in agreement with calculations. In aperiodic oxide compounds where the Ti coordination is unknown (e.g., titanosilicate glasses and melts), pre-edge position and height can be used to derive reliable information on Ti coordination chemistry. For example, one can distinguish between fivefold coordinated Ti (i.e., TiO₅) and a 50:50 mixture of fourfold- and sixfold-coordinated Ti (i.e., TiO₄ vs TiO₆). Finally, it is proposed that the intensity of the main-edge features can be used as a probe of disorder in the short- and medium-range environment of Ti. This is exemplified by Ti XANES studies of the effect of radiation damage on CaTiSiO₅ and the melting of K₂TiSi₂O₇ glass at high temperature.