Optical properties due to Cr4+ in oxides: Density functional study

Density Functional calculations on CrO44- and FeO42- tetrahedral units at d ifferent values of the metal-ligand distance, R, have been performed. Apart from explaining the experimental crystal-held and charge transfer (CT) tra nsitions (as well as the Al frequency) due to Cr4+ in Mg2SiO4 (forsterite) and Ca2GaO4 the present results indicate that the equilibrium distance for the former compound would be 6 pm smaller than in the latter case and thus the first CT transition would be placed at the end of the optical range for Cr4+ in forsterite. Absorption data in the T-3(2) crystal-field state supp ort that conclusion. As a salient feature the surprisingly small value of t he Huang-Rhys factor S-A = 0.2 observed for the 10Dq-dependent T-3(2) state of CrO44- is now reasonably accounted for. A study of the coupling with th e A(1) mode for the first CT transition and an estimation of the local bulk modulus for CrO44- are both carried out as well. (C) 1998 Elsevier Science Ltd. All rights reserved.