Resonance Raman scattering inCr4+-doped forsterite

Seven ${\mathrm{Cr}}^{4+}$ vibronic modes were obtained by means of resonance Raman scattering. These modes were measured at 253, 290, 480, 498, 690, 740, and 764 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{Cr}}^{4+}$:${\mathrm{Mg}}_2$ ${\mathrm{SiO}}_4$. The symmetry of each of the modes was established and correlated to the molecular symmetry (${\mathit{T}}_{\mathit{d}}$). The effective elastic constants for the Si-O and ${\mathrm{Cr}}^{4+}$-O bonds were estimated where a decrease of ≊13 to 29% (model dependent) was found when ${\mathrm{Cr}}^{4+}$ substitutes for Si. Possible energy matching between vibronic and host phonon modes are suggested as nonradiative pathway.