Synthesis of mixed-valent α- and β-NaFe2O3 polymorphs under controlled partial oxygen pressure

Synthesis of mixed valent compounds, especially when multiple polymorphs exist, requires careful control of the preparation conditions. α- and β-NaFe2O3 polymorphs were synthesized under controlled partial oxygen pressure (pO2). pO2 regions of stability at 850 °C were determined for both phases for the first time. A modified oxygen buffer method was developed for the facile preparation of mixed valent oxides under controlled pO2. β-NaFe2O3 is the only known n=2 member of the AMnOn+1 (A=alkali metal, M=3d metal) rock-salt related homolog series with layered cation ordering. The possibility of new members of the homolog series with other 3d metals is considered.

Graphical abstractSchematic section of phase composition vs. partial O2 pressure diagram at 850 °C for Na/Fe=1/2 and structure models of α- and β-NaFe2O3.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► α- and β-NaFe2O3 polymorphs were synthesized under controlled oxygen pressure. ► β-NaFe2O3 has rock-salt related structure with layered cation ordering. ► Existence of the rock-salt related homolog series AMnOn+1 is discussed.