A THEORETICAL-STUDY OF THE XANES SPECTRA OF RUTILE AND ANATASE

A model based on the local density approximation and on the multiple-scattering wave method has been used to analyse the experimental XANES spectra of the crystalline materials rutile and anatase. The results presented in this work are close to the best computations that can be achieved for the solid adopting a local density one-electron theory. The effect of cluster size on the computations has been analysed in the case of rutile concluding that at least 51 atoms are needed to get all the spectral features. Convergence is obtained for a model comprising 75 atoms. The agreement between the theoretical and the experimental spectra is very good even for the pre-edge structures, the origin of which has been the object of controversy. Minor differences, which could be assigned to many-body effects, have been pointed out. SCF X-alpha computations have also been performed and allow the formulation of some hypotheses concerning two-electron excitations in the XANES spectra.