Electronic Structure and Optical Properties of AFeO₂ (A = Ag, Cu) within GGA Calculations

The electronic structure of delafossite type oxides AFeO 2 (A = Ag, Cu) has been calculated using the full potential linearized augmented plane wave (FP-LAPW) method within the local spin density approximation (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 the electronic structure and the optical properties of AFeO 2 than PBE-GGA or LSDA. The influence of electron correlations has been considered within the PBE-GGA+ U and LSDA+ U methods. The effective Hubbard U, U eff, has been derived on the basis of an ab initio constraint calculation and by comparison with X-ray emission spectra. The energy band gap of AFeO 2 within the PBE-GGA+ U is found as a charge-transfer gap between O-2p to Fe-3d states. The theoretical optical band gaps Δ 0 = 1.30, Δ 1 = 2.06, and Δ 2 = 3.20 eV for CuFeO 2 are quite compatible with experimental data. We have predicted an optical band gap Δ 0 = 1.90 eV for AgFeO 2, and the increase in the optical and energy band gaps of AgFeO 2 in comparison with CuFeO 2 can be understood as a size effect.