Abstract
The compounds 
, which show high electrical conductivity, are electrochemically reduced in an aqueous alkaline solution to give rise to the oxygen deficient state 
. The electrochemical redox reactions are reversible in the range 
. The electrode potentials of
in a 
solution have been measured as a function of δ. From these data and the measurements of potential change in the cathodic processes under constant current density, the oxygen ion diffusion constants have been calculated on the basis of a linear diffusion model of oxygen ions in the crystal, which was theoretically derived. The calculated diffusion constants at 25°C are 
for 
and
, respectively. These values are very large as compared with ordinary oxides.