Comprehensive first-principles study of AgGaO₂ and CuGaO₂ polymorphs

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, r_AI=r_BIII, can provide an indication of the dispersion of the valence band of delafossite-type and ¢-NaFeO₂-type ternary oxide semiconductors.