Anomalous double-stripe charge ordering in β−NaFe2O3 with double triangular layers consisting of almost perfect regular Fe4 tetrahedra

Abstract

The physical properties of the mixed-valent iron oxide $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{NaFe}}_{2}{\mathrm{O}}_{3}$ were investigated by means of synchrotron radiation x-ray diffraction, magnetization, electrical resistivity, differential scanning calorimetry, $^{23}\mathrm{Na}$ NMR, and $^{57}\mathrm{Fe}\phantom{\rule{4pt}{0ex}}\mathrm{M}\stackrel{\ifmmode \ddot{}\else \"{}\fi{}}{\mathrm{o}}\mathrm{ssbauer}$ measurements. This compound has double triangular layers consisting of almost perfect regular ${\mathrm{Fe}}_{4}$ tetrahedra, which suggests geometrical frustration. We found that this compound exhibits an electrostatically unstable double-stripe-type charge ordering, which is stabilized by the cooperative compression of ${\mathrm{Fe}}^{3+}{\mathrm{O}}_{6}$ octahedra, owing to a valence change and ${\mathrm{Fe}}^{2+}{\mathrm{O}}_{6}$ octahedra due to Jahn-Teller distortion. Our results indicate the importance of electron-phonon coupling for charge ordering in the region of strong charge frustration.