On the nature of the lowest excited state of Cr4+-doped olivines

A Franck-Condon and ligand field analysis of published spectral data are used to characterize the lowest excited state of Cr4+ at the distorted tetrahedral sites in Ca2GeO4 as dominated by a split component of the 3T2 term, with a negligibly small 1E admixture, in contrast to Mg2SiO4 (forsterite), where a roughly 1:1 3T2 and 1E mixing is found. The small changes in the CrO equilibrium geometry on excitation, as reflected by the most prominent 0-0 line in both matrices result from the large stretching mode force constant which outweighs the vibronic force upon exciting from e2 (ground state) to e1t21 (excited state). The broad-band low-temperature emission in some Cr4+-doped olivines is assigned to bending modes which become active for sufficiently large geometric distortions.