Stoichiometric oxygen content in Nax Co O2

The crystal structure and oxygen stoichiometry in two-layer Na0.74 Co O2 and Na0.38 Co O2 at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, λ=1.1968 Å and λ=1.5403 Å, are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na0.74 Co O2 and 0.99(2) for Na0.38 Co O2, respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.