Vibrational spectra of tellurates with the garnet structure

Abstract

Abstract The IR and Raman spectra of tellurates Ln 3 Te 2 Li 3 O 12 (Ln=Pr…Lu, Y), Ca 3 Te 2 Zn 3 O 12 and Na 3 Te 2 M 3+ 3 O 12 (M 3+ = Al, Ga, Fe) with the garnet structure are presented and discussed on the basis of a group theoretical analysis, and of 6 Li— 7 Li isotopic frequency shifts. The assignment of the high-frequency bands to stretching vibrations of the TeO 6 octahedra is only an approximation whose validity depends on the nature of the tetrahedral cation (Li, Zn or M 3+ ) and of the symmetry properties of the vibrations: cationic mass effects play an important role in the antisymmetric, IR-active vibrations, whereas the bonding forces are the determining factor in the totally symmetric, Raman active vibrations. For the Na 3 Te 2 M 3+ 3 O 12 garnets, the (TeO 6 ) internal modes approximation is unacceptable since the stretching frequencies of the TeO 6 and M 3+ O 4 groups are of the same order of magnitude. No detailed assignments are available for the medium-frequency bands: they are due in part to the bending vibrations of the TeO 6 octahedra, but the contribution of the tetrahedral cation Zn 2+ or M 3+ to this region of the spectrum remains largely unknown. The translational frequencies of the dodecahedral Ln 3+ cations have been identified in the low frequency region (below 250 cm −1 ) by comparison of the frequencies of the Ln and Y garnets.