Thomas C. Brunold, Hans U. Güdel and Alexander A. Kaminskii
Optical spectroscopy of V⁴⁺ doped crystals of Mg₂SiO₄ and Ca₂GeO₄
Chem. Phys. Lett. 271, 327-334 (1997)                 

Abstract: 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 T_d C_3v C_s symmetry reduction of VO^4-₄ 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 VO₄^4- and the isoelectronic MnO₄^2- ion, this is shown to be due to efficient multiphonon relaxation from the ²T₂ LF state.