Anharmonic effects and second-neighbor interactions of local modes in Cr4+-doped forsterite probed by higher-order resonance Raman scattering

First-through-fourth-order resonance Raman spectra were measured in Cr4+: Mg2SiO4 at 77 K. Fifteen Cr4+ first-order local modes were identified from 253 to 820 cm-1. Eight modes (346, 361, 390, 408, 420, 468, 526, and 820 cm-1) were not observed in room-temperature measurements. The combinations and overtones were identified and assigned in second-through-fourth-order spectra. For a Cr4+O4 molecule nine local modes are expected in first-order spectra. The higher than expected number of first-order Cr4+ observed local modes (15 instead of 9) was explained by considering second-neighbor interactions. The analysis performed for the two tetrahedral stretching modes assigned at ω1=765 cm-1 and ω2=361 cm-1 shows that ω1 is almost insensitive to changes occurring inside the lattice (intrinsic tetrahedral mode) while the lower-frequency mode ω2 strongly responds to changes inside the lattice, acting as the link between the impurity ion and the lattice. The anharmonic constants for the 765 and 742 cm-1 local modes were estimated to be X765=-0.02 cm-1 and X742=0.08 cm-1. The importance of second-neighbor interactions for ion-lattice coupling and for nonradiative processes are discussed.