Substitutional disorder and the optical spectroscopy of gallogermanate crystals

Abstract

Results of optical absorption, photoluminescence, fluorescence lifetime and fluorescence-line-narrowing studies on several -doped crystals with the gallogermanate crystal structure are presented. Such crystals are unusual in that most fluoresce via both the broad-band transition and the sharper transition (R line), both of which are considerably broadened by the effects of substitutional disorder. An established adiabatic model is used to reproduce the selectively excited broad-band photoluminescence spectra and this shows that the substitutional disorder creates a distribution in the strength of both the octahedral crystal field and the electron - phonon coupling. The variations in electron - phonon coupling are attributed to variations in the strength of the non-octahedral crystal fields that determine the magnitudes of the coupling to non-symmetric phonon modes. This effect is shown to dominate the broadening of the band. The analysis of results of fluorescence-line-narrowing measurements on the R lines shows that these lines are also broadened by a distribution of both octahedral and non-octahedral crystal fields. In the case of crystals that contain a substitutionally disordered site in the close vicinity of the ion, the strong-field sites that give rise to R-line emission are shown to comprise an independent distribution and the broadening is dominated by the effects of the non-octahedral component of the crystal field. When the substitutionally disordered sites are further from the ion, the broad-band and R-line fluorescence derive from a single distribution of crystal fields. In this case the effect of the variation of the crystal field is again shown to be dominated by the non-octahedral contribution though the R-line fluorescence shows little evidence of this due to the strong-field sites being the extreme of the overall distribution.