Origin of the electron disproportionation in the metallic sodium cobaltates

Abstract

Recently the unusual metallic state with a substantially non-uniform distribution of a charge and mag\-ne\-tic density in CoO$_2$ planes was found experimentally in the Na$_x$CoO$_2$ compound with $x>0.6$. We have investigated an origin of such electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of a strength of the electron correlations in the $d$(Co)-shells within the GGA+U approximation for the system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sodium cations and enhanced correlations. Two important magnetic states of cobalt lattice competing with each other at realistic values of the correlation parameter were found~---~low spin hexagons (LS) and higher spin kagom\'e lattice (HS-KSL). In the heterogeneous metallic HS-KSL phase magnetic Co ions form a kagom\'e structure. In LS phase the kagom\'e pattern is decomposed into hexagons and Co ions possess minimal values of their spin. Coexistence of these states could explain the emergence of the disproportionation with the peculiar kagom\'e structure experimentally revealed in previous studies of the cobaltates.