Spontaneous Raman spectroscopy of tungstate and molybdate crystals for Raman lasers

Spontaneous Raman spectra of tungstate (MeWO4) and molybdate (MeMoO4) crystals with sheelite structure were investigated (Me = Ca, Sr, Ba, Pb). The energy shift and line broadening of internal vibrational modes were measured in the temperature range from 77 K to the melting point by means of a high temperature Raman spectroscopic technique. The phase transition from sheelite to cubic structure was discovered at a temperature of 1600 K in a BaMo04 crystal. Vibrational level diagrams for "free" [WO4](2-) and [MoO4](2-) tetrahedrons were built on the basis of spontaneous Raman spectra of tungstate and molybdate crystals in the molten state. The effect of mass and electronegativity of Me2+ cations on the position and linewidth of internal vibrational modes was investigated. It was shown that the variation of vibration level diagrams in sheelite series could change the probability of the dephasing and splitting relaxation mechanisms. As a result, the consequent line narrowing of totally symmetric Raman vibrations of [WO4] and [MoO4] tetrahedron complexes was registered in the Raman spectra in a series of sheelite crystals moving from calcium to strontium and barium. Due to the narrowest linewidth (Delta nu (R) = 1.6 cm(-1)) and the highest peak cross-section of the Raman line in barium tungstate and molybdate crystals, they were proposed as the most efficient sheelite crystals for Raman laser development. (C) 2000 Elsevier Science B.V. All rights reserved.