Comprehensive first-principles study of AgGaO₂ and CuGaO₂ polymorphs

Abstract

The electronic structures of delafossite-type α-AgGaO₂ and β-NaFeO₂-type β-AgGaO₂ were calculated based on density-functional theory using the local density approximation functional including the Hubbard correction. We compared the electronic structures of α- and β-AgGaO₂ with previously reported electronic structures of α- and β-CuGaO₂. We found that the Ag–Ag distances in α- and β-AgGaO₂ are almost the same as the Cu–Cu distances in α- and β-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 α-phase and GaO₄ tetrahedra in the β-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 α- and β-phases. We propose that the ratio of ionic radii of the monovalent and trivalent cations, r_A^I /r_B^III, can provide an indication of the dispersion of the valence band of delafossite-type and β-NaFeO₂-type ternary oxide semiconductors.