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) li gand 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 o f the angular overlap model (AOM). The onset to the lowest energy liga nd-to-metal charge-transfer transition lies in the UV above 35000 cm(- 1). Upon photoexcitation in the visible, neither of the two systems ex hibits luminescence even at cryogenic temperatures. From a comparison of the fine structure in the absorption spectra of VO44- and the isoel ectronic MnO42- ion, this is shown to be due to efficient multiphonon relaxation from the T-2(2) LF state. (C) 1997 Published by Elsevier Sc ience B.V.