50 巻 (2008) 2 号 p. 150-154
Recent works revealed that a ferroelectric polarization in the charge-frustrated system LuFe2O4 originates from an emergence of the charge ordering, which is characterized by a regular arrangement of Fe2+ and Fe3+ ions on the frustrated triangular lattice. In this work, we have investi-gated both the charge ordered (CO) structure and charge ordering processes mainly by means of the transmission electron microscopy (TEM) . Both the CO structure and charge ordering processes depend strongly on the amount of the oxygen deficiency (δ) involved in LuFe2O4-δ. In the case of δ-0, the three-dimensional CO structure with the modulation vector of q= [1/3 1/3 1/2] was formed at room temperature. On the other hand, as δ was increased, the three-dimensional CO structure changed into the two-dimensional CO one and the coherent length becomes shorter along the  direction. For proper understanding of the dielectric and magnetic properties in LuFe2O4 and related materials, it should be essential to prepare LuFe2O4-δ samples with properly controlled oxygen-concentration and elucidate the physical properties in LuFe2O4-δ with δ-0.