Stoichiometric oxygen content in NaxCoO2

TL;DR

This study uses neutron diffraction to precisely determine oxygen content in NaxCoO2, confirming near-stoichiometric oxygen levels and analyzing sodium ion geometry, which impacts cobalt oxidation states.

Contribution

The paper provides the first precise measurement of oxygen stoichiometry in NaxCoO2 using simultaneous Rietveld refinement of neutron diffraction data.

Findings

Oxygen content is stoichiometric within 1-2% accuracy.

Sodium ions are in off-center distorted trigonal prismatic geometry.

Cobalt oxidation state depends solely on sodium content.

Abstract

The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, lambda=1.1968 Angstroms and lambda=1.5403 Angstroms, 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 Na0.74CoO2 and 0.99(2) for Na0.38CoO2 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.