Charge Ordering and Dielectric Dispersion in Mixed Valence Oxides RFe2O4

In a series of oxides RFe 2 O 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. Recent developments on the structure of the ordered phases are reviewed with comments on their correlation with the observed dielectric dispersion