The research on the movement of atoms in solid body, due to thermal oscillation has important significance from the following two aspects: the knowledge of diffusion is necessary for an understanding of the properties of solid bodies at high temperatures; and from the research of diffusion it is possible to obtain useful information concerning the behavior of lattice defects in crystals, especially point defects. In the case of oxides, it is no longer as simple as in the case of simple ionic conductors, as oxides have properties of semi-conductors.