Optical absorption and Raman scattering measurements in CuAlSe2 at high pressure

The ternary compound CuAlSe2 is a direct energy gap semiconductor (E(g) = 2.6 eV at 300 K) crystallizing in the tetragonal chalcopyrite structure. In this work the optical absorption edge and the Raman active modes of CuAlSe2 were measured as a function of pressure up to 30 GPa. The measurements were performed in a membrane diamond-anvil cell at ambient temperature using neon gas as pressure transmitting medium. The direct energy gap (I-4(v) --> I-1(c)) increases linearly with pressure at the rate of 4.7 x 10(-2) eV GPa(-1). At 6.7 GPa the character of the fundamental gap changes to pseudodirect (I-4(v) --> I-3(c)). This gap decreases with pressure at a rate of -2.9 x 10(-2) eV GPa(-1). The effect of pressure on the phonon frequencies is discussed in terms of the Gruneisen parameters. A first-order structural phase transition was observed at 12 GPa in the upstroke and at 2 GPa in the downstroke.