Investigations of the optical spectral bands, g factors and local structure for the tetragonal Cr~(5+) tetrahedral clusters in Cr-doped silica glasses

摘要

In this paper, the weaknesses and mistakes in the assignments of some optical absorption bands and the theoretical calculations of g factors (g// < g_⊥) in the previous papers for the tetragonally-elongated (CrO_4)~(3-) clusters in Cr-doped silica glasses prepared with sol-gel method are pointed out and corrected. On the basis of these, the g factors g// and g_⊥ for this Cr~(5+) cluster are calculated from the high-order perturbation formulas based on the two-mechanism model (where the contributions to g factors due to both the crystal field (CF) mechanism in the widely-used CF theory and the charge-transfer (CT) mechanism are included) for the tetragonally-elongated d~1 tetrahedral clusters with the ground state |d_(x-y)~(22)). From the calculated values and by considering the small admixture between the ground state |d_(x-y)~(22)) and the first excited state|d_z~2) owing to the vibrational motion of ligands (note: this dynamic effect leads the elongated (CrO_4)~(3-) cluster to become the twinkling compressed one), the calculated g// and g_⊥ are in reasonable agreement with the experimental values. In the calculations, the needed CF energy levels (corresponding to the optical spectral bands) are obtained from CF theory in which the CF parameters are estimated from the superposition model. Thus, the optical band positions of (CrO_4)~(3-) cluster in silica glassed are explained and the local structure of static (CrO_4)~(3-) cluster is obtained. By considering the dynamic effect, the isotropic g factors (g// = g_⊥) at room temperature for another (CrO_4)~(3-) cluster in silica glasses and the anisotropic g factors (g//>g_⊥) for the tetragonally-compressed (CrO_4)~(3-) cluster in barium aluminophosphate glasses are also explained in a unified way. The results are discussed.