Stoichiometric oxygen content in NaxCoO2

The crystal structure and oxygen stoichiometry in two-layer Na_0.74CoO₂ and Na_0.38CoO₂ at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, λ=1.1968Å and λ=1.5403Å , 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 Na_0.74CoO₂ and 0.99(2) for Na_0.38CoO₂ , 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.