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.

The influence of hydrostatic pressure on the Raman scattering spectra in e- GaSe (up to 8.5 kbar) and ZnIn,S, (up t o 10.9 kbar) is investigated a t 295 K. Parameters Y{ = (l/vc) (dv,/dp) are obtained for both the single crystals; for E-GaSe values of the mode-Gruneisen parameters are also obtained. The set of the parameters ye(r,) shows that these two crystals are layered ones with ionic-covalent bonds inside the layers; the degree of ionicity in ZnIn,S, is higher than in c-GaSe. "Rigid-layer" (interlayer) vibrations determined for ZnIn,S, and E-GaSe using yi-parameters show that ZnIn,S, is not a one-packet polytype.

No abstract

No abstract

The spectra of the absorption coefficient (a) of GaSe a t the photon energy range 1.94 t o 2.64 eV and a t pressures between 1 bar and 12 kbar are investigated a t 295 O K . Data a t 1 t o 4360 bar confirm that the edge exciton of GaSe is the direct allowed one. The pressure coefficient of the band gap (dE,/dP = -6.2 x eV bar-I), one of the Rydberg exciton constant (dR/dP = -0.6 x eV bar-*), and one of the exciton effective massIt is supposed that two interband exciton transitions a t 2.001 and 3.25 eV give the contribution into the a spectrum above 2.1 eV. The assignment of these transitions is discussed in connection with the band structure calculation of Schluter. At 5280 bar a discontinuous jump of the absorption edge shape is observed for GaSe, with a change of t h e sign and a value for dE,/dP of +2.0 x 10-6 eV bar-' within the range of 5280 t o 11780 bar. This effect is interpreted as the transformation of one GaSe polytype into another, i.e. as a possible phase transformation close to t h a t of second kind. B HHTepBane 3~e p r~f i @oToHa OT i,94 no 2,64 eV y1 DaBneHHR OT 1 6ap no 12 kbar npH 295 O K accnenoBaHx cnempbI K O~@ @~U M~H T~ n o r n o q e~n~~ a ceneHana r a n n m GaSe. JJaHHbIe B ~n a n a a o~e OT 1 AO 4360 bar nonTsepmaamT, TO ~K C H T O H Ha KpaIo IIOrJIOIIJeHHFI GaSe RBJIReTCR IIpFIMbIM pa3peUIeHHbIM 3IECHTOHOM. npH6JIPiweHHO OIrpe~eneHbI K03@@Pi4HeHTbI HaBneHHR IUllpllHbI a a n p e q e~~o a 30HbI (dEg/dP = ---6,2 x eV -bar-I), ~C