Lattice dynamics ofAgGaSe2. I. Experiment

Abstract

We report a detailed investigation of the infrared and Raman activity of ${\mathrm{AgGaSe}}_2$. Performing, first, infrared studies in the spectral range 15–500 ${\mathrm{cm}}^{\mathrm{-}1}$, both at room and at liquid-helium temperature, we could identify all ${\mathrm{\Gamma }}_4$ (E∥C ) and ${\mathrm{\Gamma }}_5$ (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 ${\mathrm{\Gamma }}_5$ doublet at 249.5–252.1 ${\mathrm{cm}}^{\mathrm{-}1}$. 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${\mathrm{\Gamma }}_1$ and 3${\mathrm{\Gamma }}_3$. 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 ${\mathrm{cm}}^{\mathrm{-}1}$ are four modes (1${\mathrm{\Gamma }}_3$+1${\mathrm{\Gamma }}_4$+2${\mathrm{\Gamma }}_5$); next, in an intermediate region extending from 180 to 135 ${\mathrm{cm}}^{\mathrm{-}1}$ are five modes (1${\mathrm{\Gamma }}_1$+1${\mathrm{\Gamma }}_3$+1${\mathrm{\Gamma }}_4$+2${\mathrm{\Gamma }}_5$); and, finally, at much lower energy the dispersion of the acoustic branches gives, in the range 25–85 ${\mathrm{cm}}^{\mathrm{-}1}$, another series of four modes which are associated with 1${\mathrm{\Gamma }}_3$+1${\mathrm{\Gamma }}_4$+2${\mathrm{\Gamma }}_5$ symmetry.