Zero-wave-vector optical phonons in AgGaS₂ reexamined

The identification of first- and second-order features observed near zero wave vector in the Raman spectrum of AgGaS₂ is reexamined in view of a recent study of its optical properties with terahertz spectroscopy and temperature-dependent Raman scattering (H. Qiao et al., Optical Materials 119, 111300, 2021). An assignment of the weak Raman peak at 35.5 cm⁻¹ to the lowest frequency E-symmetry mode from its temperature dependence is confirmed by the spectra obtained from terahertz spectroscopy. Another strong Raman peak of E symmetry is shown to arise from second-order combination scattering due to its pronounced abnormal temperature dependence. It is concluded that discerning the peaks of interest amongst a plethora of Raman, or infrared, features can be aided by low temperature measurements where the peaks are sharper and second-order features are much reduced in intensity.