Abstract
We report a detailed investigation of the infrared and Raman activity of . Performing, first, infrared studies in the spectral range 15–500 , both at room and at liquid-helium temperature, we could identify all (E∥C ) and (E⊥C ) vibrational modes predicted by group-theory arguments. This includes, at low energy, the complete series of folded-acoustic components resolved for the first time and second, at higher energy, a nearly degenerate doublet at 249.5–252.1 . All frequencies obtained in that way were independently checked by performing, under nearly resonant conditions, Raman investigations at 77 K. We could then identify four additional modes: 1 and 3. Our experimental results demonstrate that all zone-center modes can be grouped in three energy bands: First, within a very narrow energy range centered around 250 are four modes (1+1+2); next, in an intermediate region extending from 180 to 135 are five modes (1+1+1+2); and, finally, at much lower energy the dispersion of the acoustic branches gives, in the range 25–85 , another series of four modes which are associated with 1+1+2 symmetry.
- Received 16 August 1989
DOI:https://doi.org/10.1103/PhysRevB.41.5717
©1990 American Physical Society