Comprehensive first-principles study of AgGaO2 and CuGaO2 polymorphs

Abstract

The electronic structures of delafossite-type ¡-AgGaO2 and ¢-NaFeO2-type ¢-AgGaO2 were calculated based on density-functional theory using the local density approximation functional including the Hubbard correction. We compared the electronic structures of ¡- and ¢-AgGaO2 with previously reported electronic structures of ¡- and ¢-CuGaO2. We found that the AgAg distances in ¡- and ¢-AgGaO2 are almost the same as the CuCu distances in ¡- and ¢-CuGaO2, respectively, despite the ionic radius of Ag+ being larger than that of Cu+, because the frameworks of their crystal structures are determined by the linkages of GaO6 octahedra in the ¡- phase and GaO4 tetrahedra in the ¢-phase. It is indicated that transfer of electrons in Ag 4d states of AgGaO2 can occur between Ag atoms more easily than in Cu 3d states of CuGaO2 because high-electron-density regions surrounding Ag atoms are closer to each other than those of Cu atoms. This resulted in larger dispersion of the valence band of AgGaO2 than CuGaO2 for both ¡- and ¢-phases. We propose that the ratio of ionic radii of the monovalent and trivalent cations, rAI=rBIII, can provide an indication of the dispersion of the valence band of delafossite-type and ¢-NaFeO2-type ternary oxide semiconductors.