We report a detailed investigation of the infrared and Raman activity of AgGaSe2. Performing, first, infrared studies in the spectral range 15500 cm-1, both at room and at liquid-helium temperature, we could identify all 4 (EC) and 5 (EC) 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 5 doublet at 249.5252.1 cm-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: 11 and 33. 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 cm-1 are four modes (13+14+25); next, in an intermediate region extending from 180 to 135 cm-1 are five modes (11+13+14+25); and, finally, at much lower energy the dispersion of the acoustic branches gives, in the range 2585 cm-1, another series of four modes which are associated with 13+14+25 symmetry. © 1990 The American Physical Society.
|Journal||Physical Review B|
|Publication status||Published - 1 Jan 1990|