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 E-2 --> T-2(2) ligand field (LF) transition peaks in the near infrared around 12000 cm(-1). The spectra are interpreted in terms of a stepwise T-d --> C-3v --> C-s 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 T-2(2) LF state. (C) 1997 Published by Elsevier Science B.V.