Raman scattering and phase transformations of TlGaSe 2 and TlGaS 2 under pressure

Raman spectra of single crystals TlGaS 2 and TlGaSe 2 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 γ i = (1/ v i ) (d v i /d P ) 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 TlGaS 2 and TlGaSe 2 is supposed to contain two layers and the structure of the layer is supposed to be similar to that of TlSe. [Russian Text Ignored].