Effect of Cr substitution on the electronic structure of CuAl _1-xCr_xO₂

The geometries and electronic structures of CuAl1-x Crx O2 have been investigated using density-functional theory with on-site corrections for strongly correlated systems (GGA+U) for x=0, 1 2, 1. Al is found to be well described within the ionic model, with a valence charge corresponding to a +3 oxidation state. Substituting Cr for Al is predicted to increase the density of states at the top of the valence band, in agreement with experimental x-ray photoemission spectroscopy data. Analysis of atom-projected densities of states and valence charges suggests that this is due to increased covalency between Cr and O; the valence charge for O in the Cu-O-(Al,Cr) subunits changes from -1.74 to ∼-1.25 when Cr replaces Al. This produces an indirect oxygen-mediated change to the Cud states.