  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.
