Absence of conventional charge ordering in Na0.5CoO2 from a high resolution neutron diffraction study

The structure of Na0.5CoO2, the low temperature insulator that separates the magnetic and superconducting regions in the NaxCoO2.yH2O phase diagram, is studied by high resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in one-dimensional chains in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites with different numbers of Na neighbours, but the difference in their Co-O bond distances and the derived bond valence sums is small.