Influence of Low-Symmetry Distortions on the Luminescence of ${\mathrm{Cr}}^{4+}$-Doped Forsterite

By using pressure to vary the extent of nontetrahedral distortions of the ${\mathrm{Cr}}^{4+}$ site in ${\mathrm{Mg}}_2{\mathrm{SiO}}_4$, we reveal the important influence of the ${}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}$ state on the emission properties of the ${}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}$ state. We find that ${}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}$ mixing has a pronounced effect on the line shape and radiative decay rate of emission from the ${}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}$ state and that the extent of mixing depends critically on the magnitude of nontetrahedral distortions. The results provide an explanation for the wide variation of ${\mathrm{Cr}}^{4+}$ emission properties in different host lattices at ambient pressure and indicate that the tailoring of asymmetric distortions of luminescent centers represents an effective new strategy for tuning the linewidth of spectral transitions.