Raman scattering and phase transformations of TlGaSe2 and TlGaS2 under pressure

Abstract
Raman spectra of single crystals TlGaS2 and TlGaSe2 are investigated in equilibrium conditions at room temperature under various hydrostatic pressures up to 7.25 and 9.27 kbar, respectively. Phase transitions at about 5 kbar are shown to occur in both the single crystals. Mode parameters γi = (1/vi) (dvi/dP) are determined for most of the Raman bands observed. The different values of these parameters are used to find out which frequencies have to be assigned to interlayer vibrations. It is shown that Raman spectra can be explained best when the unit cell of TlGaS2 and TlGaSe2 is supposed to contain two layers and the structure of the layer is supposed to be similar to that of TlSe.