CuGa( SxSe1 − x) 2 alloys at high pressure: Optical absorption and X-ray diffraction studies

Abstract

The CuGa( S x Se 1 − x ) 2 semiconductor shows a single phase (chalcopyrite) solid solution over the whole composition range and a continuous variation of structure and electronic properties. The optical absorption edge of the single crystals was measured as a function of hydrostatic pressure, covering the full stability range (0–30 GPa) of the chalocopyrite phase. For the full range 0 ⩽ x ⩽ 1, the direct energy gap exhibits linear increase under pressure, with the corresponding gap deformation potential ( a g ) being − 4.5 eV for x = 0, − 5.2 eV for x = 0.5 and − 4.4 eV for x = 1. For pressures higher than a certain value denoted by P t , (optical) all the samples become opaque (transmission less than 0.03% in the spectral range 0.7–2.8 eV). We presume that a phase transformation to a metallic (or narrow gap semiconductor) structure has occurred. In situ high-pressure energy-dispersive X-ray diffraction studies were carried out in the alloy series (0 ⩽ x ⩽ 1) up to 35 GPa at the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique (LURE, Orsay, France). Pressure-induced first-order phase-transitions from the chalcopyrite structure to the rock-salt structure were observed at P t = 14.5 GPa for x = 0, P t = 15.7 GPa for x = 0.5 and P t = 18 GPa for x = 1. The transitions are structurally reversible and take place without amorphization. The hysteresis of the transition has also been studied. A Murnaghan equation of state fitted to the values of the cell parameters in the chalcopyrite phase gives the values of the bulk modulus.