Successive Transition of Polaron Phases in Triangular Charge Competing System YFe_2O_4

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In a series of oxides RFe_2O_4 (R=rare earth between Dy and Lu, or Y), equal numbers of Fe^<2+> and Fe^<3+> are located on a layer composed of two triangular nets stacked together. These oxides have several phase transitions where the arrangement of Fe^<2+> and Fe^<3+> ions transforms from one type to another. Unique dielectric properties of these oxides have been explained in connection with the charge ordering.We made a research of low oxygen deficient YFe_2O_4 in which the charge frustration effect is considered to be dominant. We gave a discussion that the possible origin of the successive transition of the superstructure arose from the competition between the nearest neighbor and next nearest neighbor interaction of charges in triangular plane. We had synthesized the low oxygen deficient YFe_2O_4 with a furnace which equipped an electric circuit system which control an oxygen partial pressure. A synchrotron radiation X-ray diffraction, transmission electron microscope experiment r … More evealed that, stoichiometric YFe_2O_4, the superstructure of the Fe^<2+> and Fe^<3+> order transformed into a triple, double, quadruple and septenary charge structure in the temperature range from room temperature to 130 K. A slow relaxation of the superstructure was found at 130 K. Time dependence of the magnetic moment was observed at about 240 K to suggest a similar relaxation.By a transmission electron microscopy observation for YFe_2O_4, a characteristic diffuse scattering elongated along the [001] direction through the (1/3 1/3 0)-type reciprocal positions was found at RT, which is related to the micro domains due to the CO with a size of 10-20nm. In addition, it was also found that the superlattice structures characterized by (1/3-・1/3-・0)-type modulation vector, which should be due to the vacancy ordering, coexist with the microdomains due to the CO. On the other hand, in stoichimetric YFe_2O_4, we found temperature variations of the superlattice structures on warming from 100 K, which should be strongly related to the dielectric anomalies in YFe_2O_4.A resonant X-ray diffraction revealed that the charge superstructure is originated from the long range ordering of Fe^<2+> and Fe^<3+> in triangular plane.In addition to the above researches we made a structure analysis of topological crystal. And we made some researches in which a coupling between charge and spin of transition metal d-electron plays a central role to drives a lattice distortion or phase transition. Less