α-NaFeO type layered oxides shows reversible intercalation/de-intercalation of sodium ions between the oxide layers, which is suitable as regenerable CO absorber and cathode material for secondary batteries. This paper reports a critical effect of heating atmosphere to synthesize pure α-NaFeO from equimolar sodium nitrate and iron oxide. Reaction of NaNO with γ-Fe O was much faster in nitrogen than oxygen atmosphere and high-purity α-NaFeO was yielded in a few hours of heat treatment in nitrogen while in oxygen atmosphere the reaction was sluggish, resulting in partially sodiated γ-Fe O . Inert atmosphere thermodynamically enhances nitrate decomposition, which controls the reaction rate of NaNO with γ-Fe O , and increases the solubility of γ-Fe O in molten NaNO to promote topotactic phase transformation to α-NaFeO . Molten NaNO works as a solvent for ferric ions and a redox buffer for maintaining the lattice oxygen structure and oxidation state of γ-Fe O . Larger iron oxide crystal was found to be suitable for production of high-purity α-NaFeO in a wide temperature range. 2 2 2 3 2 3 2 2 3 3 2 3 2 3 3 2 3 2 3 2