| Abstract
| - The electronic structure of delafossite type oxides AFeO2 (A = Ag, Cu) has been calculated using thefull potential linearized augmented plane wave (FP-LAPW) method within the local spin densityapproximation (LSDA), Perdew−Burke−Ernzerhof (PBE-GGA), and Engel−Vosko (EV-GGA) generalized-gradient approximations. It was found that the EV-GGA provides a more realistic description of theelectronic structure and the optical properties of AFeO2 than PBE-GGA or LSDA. The influence ofelectron correlations has been considered within the PBE-GGA+U and LSDA+U methods. The effectiveHubbard U, Ueff, has been derived on the basis of an ab initio constraint calculation and by comparisonwith X-ray emission spectra. The energy band gap of AFeO2 within the PBE-GGA+U is found as acharge-transfer gap between O-2p to Fe-3d states. The theoretical optical band gaps Δ0 = 1.30, Δ1 =2.06, and Δ2 = 3.20 eV for CuFeO2 are quite compatible with experimental data. We have predicted anoptical band gap Δ0 = 1.90 eV for AgFeO2, and the increase in the optical and energy band gaps ofAgFeO2 in comparison with CuFeO2 can be understood as a size effect.
- First-principles calculations using Engel−Vosko−GGA reveal a semiconducting state for AgFeO2 and CuFeO2. We used the PBE-GGA+U method to verify the band gaps. An effective U of 2.18 eV was derived. We predict an optical band band gap Δ0 = 1.90 eV for AgFeO2. For CuFeO2, one optical band gap Δ0 = 1.30 eV and two optical gaps Δ1 = 2.06 and Δ2 = 3.20 eV have been derived, which are in good agreement with experimental report. The energy band gaps of CuFeO2 and AgFeO2 are considered to be the charge-transfer gap.
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