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The Raman scattering spectra of CdGa 2 Se 4 under pressure were investigated at 300 K up to 20.9 GPa. TO as well as LO modes were observed. Two stages were observed in the pressure dependences of Raman bands. Such a behaviour was attributed to the order -disorder phase transition in the cation sublattice. A first-order phase transition under pressure was observed at 20.2 GPa. Using the Harrison -Keating model of the lattice dynamics modified for crystals with the tetragonal structure, the bulk modulus B and the mode-Grüneisen parameters Γ i were determined for the first time. It is shown that a better agreement between the experimental and calculated values of Γ i is observed if one takes into consideration the different behaviour with pressure for the bond-bending and the bond-stretching parameters, which determine the low-(lower than 140 cm -1 ) and high-(higher than 140 cm -1 ) frequency phonons, respectively.

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