Ab initio study of structural, bonding, and vibrational properties of AGaO2 (A = Ag, Cu) delafossites

We have performed an ab initio study of structural, bonding, and vibrational properties of AgGaO2 and CuGaO2 delafossites. The calculations have been carried out within the density functional theory and linear response formalism using ultrasoft pseudopotentials and a plane wave basis. Structural properties of AgGaO2 and CuGaO2 are in very good agreement with experimental data, and zone centre phonons in CuGaO2 are in excellent agreement with experimental and previous DFT calculation. Two Raman-active modes, Eg and A1g in AgGaO2 are found to have frequencies at 367 cm−1 and 682 cm−1 respectively. A noticeable difference between these calculated and experimentally observed Raman-active modes have been found. Calculated Raman-active modes have been further verified using atomic-orbital basis and norm-conserving pseudopotentials within finite displacement frozen phonon technique. We also present the phonon dispersion relations, phonon density of states, pressure-coefficients, and mode-Grüneisen parameters at high symmetry points in these oxides. Our DFT calculations predict dynamical instability, possibly related to phase transition around 17 GPa in AgGaO2 delafossite.