In this letter, we discuss the electronic structure of copper aluminate ͑CuAlO 2 ͒ on the basis of absorption measurements at low temperature and under high pressure in single crystals and thin films, combined with ab initio electronic structure calculations. The indirect character of the fundamental transition could be clearly established through the photon energy dependence of the absorption edge as measured in single crystals, yielding a band gap ͑plus a phonon͒ of 2.99± 0.01 eV at room temperature. An indirect excitonic structure is also observed at low temperature. The lowest energy direct allowed transition occurs near 3.53± 0.01 eV and is identified through its low-energy tail in single crystals and its excitonic peak in thin films. Comparing our results with ab initio calculations available in the literature, we conclude that CuGaO 2 and CuInO 2 can hardly be considered as wide gap semiconductors, even if they are transparent in thin films.

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The density functional perturbation theory is employed to study the vibrational properties of CuAlO 2 under pressure. The calculations are preformed using the pseudopotential wave method and the local density approximation for the exchange-correlation (XC) potential. The d electrons of Cu are treated as valence states. We present the phonon dispersion curves. Our results are in good agreement with the available experimental Raman scattering experiments. Ab initio calculations show the presence of a dynamical instability, possibly related with the experimentally observed phase transition.