Electronic structure of CuAIO(2) and CUScO2 delafossites under pressure

The electronic structure of CuAlO2 and CuScO2 delafossites is investigated by means of optical absorption measurements under pressure and ab initio band structure calculations. Measurements are carried out on CuAlO2 monocrystals and pulsed laser deposited CuAlO2 and CuScO2 thin films up to 20 GPa. CuAlO2 is an indirect semiconductor that is stable in the pressure range explored here. The pressure coefficients of the indirect and direct gaps are found to be 15 meV/GPa and 2 meV/GPa respectively. CuScO2 is a direct semiconductor and the pressure coefficient of the excitonic peak energy is -5.5 meV/GPa. Two reversible phase transitions are observed in CuScO2. At 13 GPa the delafossite structure becomes unstable and an unidentified intermediate high pressure phase is observed, which coexists with the delafossite phase up to 18 GPa. The intermediate phase is also a wide gap semiconductor, with an step-like absorption edge at the energy of 3.42 +/- 0.02 eV at 14.0 GPa and a positive pressure coefficient. Above 18 GPa, the absorption edge becomes structureless and its form seems to be that of an indirect gap. In the downstroke, the same sequence of changes is observed, with a hysteresis of about 4 GPa, confirming the structural nature of the transitions. These results are interpreted in the light of theoretical ab initio band structure calculations.