Anharmonic Effects and Second-Neighbor Interactions of Local Modes in Cr(4+)-Doped Forsterite Probed by Higher-Order Resonance Raman Scattering

摘要

First-through-fourth-order resonance Raman spectra were measured inCr(4+):Mg2SiO4 at 77 K. Fifteen Cr(4+) first-order local modes were identified from 253 to 820 /cm. Eight modes (346, 361, 390, 408, 420, 468, 526, and 820 /cm) were not observed in room-temperature measurements. The combinations and overtones were identified and assigned in second-through-fourth-order spectra. For a Cr(4+)O4 molecule nine local modes are expected in first-order spectra. The higher than expected number of first-order Cr(4+) 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 and omega 2=361 /cm shows that omega 1 is almost insensitive to changes occurring inside the lattice (intrinsic tetrahedral mode) while the lower-frequency mode omega 2 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 local modes were estimated to be X sub 765=-0.02 /cm and X sub 742=0.08 /cm. The importance of second-neighbor interactions for ion-lattice coupling and for nonradiative processes are discussed.