title: First-Principles Study of CuGaO2 Polymorphs: Delafossite α-CuGaO2 and Wurtzite β-CuGaO2.
abstract: The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of cations. In the conduction band of α-CuGaO2 with a layered structure of Cu2O and Ga2O3, Cu and Ga states do not mix well; the lower part of the conduction band mainly consists of Cu 4s and 4p states, and the upper part consists of Ga 4s and 4p states. By contrast, in β-CuGaO2, which is composed of CuO4 and GaO4 tetrahedra, Cu and Ga states are well-mixed. The second feature is the coordination environment of Cu atoms; the breaking of degeneracy of Cu 3d orbitals is determined by the crystal field. Dispersion of the Cu 3d valence band of β-CuGaO2, in which Cu atoms are tetrahedrally coordinated to oxygen atoms, is smaller than those in α-CuGaO2, in which Cu atoms are linearly coordinated to oxygen atoms; this results in a larger absorption coefficient and larger hole effective mass in β-CuGaO2 than in α-CuGaO2. The interatomic distance between Cu atoms-the third feature-also influences the dispersion of the Cu 3d valence band (i.e., the effective hole mass); the effective hole mass decreases with decreasing interatomic distance between Cu atoms in each structure. The results obtained are valuable for understanding the physical properties of oxide semiconductors containing monovalent copper and silver.
authors.affiliation: Division of Material and Manufacturing Science, Graduate School of Engineering, Osaka University , Suita, Osaka 565-0871, Japan.
authors.forename: Issei
authors.initials: I
authors.lastname: Suzuki
authors.affiliation: Division of Material and Manufacturing Science, Graduate School of Engineering, Osaka University , Suita, Osaka 565-0871, Japan.
authors.forename: Hiraku
authors.initials: H
authors.lastname: Nagatani
authors.affiliation: Department of Mechanical Engineering, National Institute of Technology, Toyama College , Toyama 939-8630, Japan.
authors.forename: Masao
authors.initials: M
authors.lastname: Kita
authors.affiliation: Interdisciplinary Graduate School of Medical & Engineering Material Science and Technology, University of Yamanashi , Kofu, Yamanashi 400-8510, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Iguchi
authors.affiliation: Interdisciplinary Graduate School of Medical & Engineering Material Science and Technology, University of Yamanashi , Kofu, Yamanashi 400-8510, Japan.
authors.forename: Chiyuki
authors.initials: C
authors.lastname: Sato
authors.affiliation: Interdisciplinary Graduate School of Medical & Engineering Material Science and Technology, University of Yamanashi , Kofu, Yamanashi 400-8510, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yanagi
authors.affiliation: National Institute for Materials Science , Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Ohashi
authors.affiliation: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University , Katahira, Aoba-ku, Sendai 980-8577, Japan.
authors.forename: Takahisa
authors.initials: T
authors.lastname: Omata