Lattice vibrations of AgGa${\mathrm{S}}_2$, AgGa${\mathrm{Se}}_2$, and CuGa${\mathrm{S}}_2$

Raman spectra have been observed in AgGa${\mathrm{S}}_2$, AgGa${\mathrm{Se}}_2$, and CuGa${\mathrm{S}}_2$ using argon, krypton, and dye lasers. Of the 13 Raman-active vibrations, we have observed 12 of the modes of AgGa${\mathrm{S}}_2$, 9 modes of AgGa${\mathrm{Se}}_2$, and all the modes of CuGa${\mathrm{S}}_2$. The modes of AgGa${\mathrm{S}}_2$ which are also infrared active are in good agreement with the modes determined from a Kramers-Kronig analysis of the ir reflectivity. Agreement is also found with several of the transitions previously determined from the ir spectrum of CuGa${\mathrm{S}}_2$. The intensity of the $A_1$ mode of AgGa${\mathrm{S}}_2$ and CuGa${\mathrm{S}}_2$ dominates the other Raman lines when the laser excitation is well below the band gap. A resonant interference effect decreases the intensity of this mode as the band gap is approached.