Charge Delocalization, Oxidation States, and Silver Mobility in the Mixed Silver-Copper Oxide AgCuO₂ ¤

The electronic structure of AgCuO₂, and more specifically the possible charge delocalization and its implications for the transport properties, has been the object of debate. Here the problem is faced by means of first-principles density functional theory calculations of the electron and phonon band structures as well as molecular dynamics simulations for different temperatures. It is found that both Cu and Ag exhibit noninteger oxidation states, in agreement with previous spectroscopic studies. The robust CuO₂ chains impose a relatively short contact distance to the silver atoms, which are forced to partially use their d _z orbitals to build a band. This band is partially emptied through overlap with a band of the CuO₂ chain, which should be empty if copper were in a Cu³⁺ oxidation state. In that way, although structural correlations could roughly be consistent with an Ag⁺Cu³⁺O₂ formulation, the appropriate oxidation states for the silver and copper atoms become Ag^(1+δ)+ and Cu^(3-δ)+, and as a consequence, the stoichiometric material should be metallic. The study of the electronic structure suggests that Ag atoms form relatively stable chains that can easily slide despite the linear coordination with oxygen atoms of the CuO₂ chains. Phonon dispersion calculations and molecular dynamics simulations confirm the stability of the structure although pointing out that sliding of the silver chains is an easy motion that does not lead to substantial modifications of the electronic structure around the Fermi level and, thus, should not alter the good conductivity of the system. However, this sliding of the silver atoms from the equilibrium position explains the observed large thermal factors.