Charge frustration in the mixed-valent iron oxide β-NaFe₂O₃ with a double triangular layer consisting of Fe₄ tetrahedra

We have investigated physical properties of β-NaFe₂O₃ with mixed-valent Fe^2.5+ ions. This compound has double triangular layers (W layers) of Fe ions similar to LuFe₂O₄, and however, the coordination number of Fe ions of these two compounds is different; the coordination number is six and five for β-NaFe₂O₃ and LuFe₂O₄, respectively. The W layers consist of the two-dimensional network of almost regular Fe₄ tetrahedra. Thus, charge frustration is expected to be large in β-NaFe₂O₃. However, there are no reports on physical properties of β-NaFe₂O₃ to our knowledge_, although there are many reports on the synthesis of this compound.

In this talk, we will report anomalous charge ordering phenomena in β-NaFe₂O₃. The charge ordering phenomena were investigated by means of magnetic, electrical, nuclear magnetic resonance, and synchrotron X-ray diffraction measurements. This compound shows charge and magnetic orderings at different temperature. Interestingly, this compound exhibits an electrostatically unstable CO pattern, which does not satisfy the Anderson's condition. We will discuss the effect of charge frustration and the role of electron-phonon interactions and Coulomb interactions.