Optical spectroscopy of V4+ doped crystals of Mg2SiO4 and Ca2GeO4

Polarized low-temperature absorption spectra of V4+ doped crystals of Mg2SiO4 and Ca2GeO4 are presented and discussed. The 2E → 2T2 ligand field (LF) transition peaks in the near infrared around 12000 cm−1. The spectra are interpreted in terms of a stepwise Td → C3v → Cs symmetry reduction of VO44− and analyzed within the framework of the angular overlap model (AOM). The onset to the lowest energy ligand-to-metal charge-transfer transition lies in the UV above 35000 cm−1. Upon photoexcitation in the visible, neither of the two systems exhibits luminescence even at cryogenic temperatures. From a comparison of the fine structure in the absorption spectra of VO44− and the isoelectronic MnO42− ion, this is shown to be due to efficient multiphonon relaxation from the 2T2 LF state.