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 omega(1) = 765 cm(-1) and omega(2) = 361 cm(-1) shows that omega(1) is almost insensitive to changes occurring inside the lattice (intrinsic tetrahedral mode) while the lower-frequency mode wt 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 X(765) = -0.02 cm(-1) and X(742) = 0.08 cm(-1). The importance of second-neighbor interactions for ion-lattice coupling and for nonradiative processes are discussed.