In order to elucidate the Stark structure of ${\mathrm{Tm}}^{3+}$ multiplets involved in the ≊2 μm emission of YAG:${\mathrm{Tm}}^{3+}$, and YAG:${\mathrm{Cr}}^{3+}$,${\mathrm{Tm}}^{3+}$, absorption and emission spectra have been measured over a wide range of concentrations. The ${\mathrm{Tm}}^{3+}$ spectra in YAG show, besides the main lines assigned to dodecahedral ${\mathit{D}}_2$ sites, a set of satellites associated with ${\mathrm{Tm}}^{3+}$ in perturbed sites. An attempt to correlate ${\mathrm{Tm}}^{3+}$ spectra with YAG structure and structural defects has been made. Using our data and an improved algorithm we have been able to identify Stark levels of ${\mathrm{Tm}}^{3+}$ in ${\mathit{D}}_2$ (${\mathrm{Y}}^{3+}$) sites, especially in the ${}_{}{}^3{}_{}{}^{}$${\mathit{F}}_4$, ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$, ${}_{}{}^3{}_{}{}^{}$${\mathit{F}}_3$, and ${}_{}{}^3{}_{}{}^{}$${\mathit{F}}_2$ multiplets. The spectra due to ${\mathrm{Tm}}^{3+}$ in perturbed sites, with a lower symmetry, were used to assign levels corresponding to forbidden transitions in the ${\mathit{D}}_2$ symmetry. Some of the identified levels in ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$ can explain the luminescence data and the cross-relaxation mechanism that lead to the population of the ${}_{}{}^3{}_{}{}^{}$${\mathit{F}}_4$ metastable level, while those in ${}_{}{}^3{}_{}{}^{}$${\mathit{F}}_3$ seem to play an important role in ${\mathrm{Cr}}^{3+}$-${\mathrm{Tm}}^{3+}$ energy transfer. In ${\mathrm{Cr}}^{3+}$-codoped samples supplementary lines in both ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ spectra as well as a preferential energy transfer have been observed and explained in terms of structural data.

• Received 16 December 1992

DOI:https://doi.org/10.1103/PhysRevB.47.14084