Phase Transition of Li2O-Fe2O3 System. III. Long-range Order in Lithium Ferrite and Lithium Ferrospinel

Abstract

In the previous papers1,2), the thermal and the electric properties of lithium fer-rite and lithium ferrospinel were studied from the viewpoint of an order-disorder transition. Then the anomalous phenomena in these properties were observed at the transition. The order-disorder transition in binary alloys has been widely studied by the statistical thermodynamics2). In order to specify the degree of order for the arrangement of atoms, two kinds of order parameters are defined. One is a long-range parameter introduced by Bragg and Williams, and the other is a short-range order parameter introduced by Bethe. When the former parameter is present, some super-stucture reflections appear in an X-ray diffraction. At the transition temperature, this parameter vanishes and the latter parameter remains in a diffuse scattering of X-rays. The long-range order parameter in a substitutional transition, which was defined by Buerger4), has not yet been measured except of binary alloys. As the first example of an extension of the transition theory for binary alloys to more complex crystals, temperature dependencies of the long-range order parameter for lithium ferrite and lithium fer-rospinel were measured by an X-ray powder method.