Defect Concentration Relaxation During Interdiffusion Between MnO and FeO

Abstract

AbstractFor the diffusional equilibration process between crystals of MnO and FeO it was investigated how far the usual assumption of local thermodynamic point defect equilibrium holds (given the intensive variables, T, P, and μo2 (gas)). To this end, accurate component concentration profiles which were normalized for the given linear diffusion geometry with t, have been determined. From the fact that these profiles do not coincide for different diffusion times it is concluded that deviations from the local point defect equilibrium occur. The largest deviations are found at the beginning of the interdiffusion process. The point defect relaxation (cation vacancy annihilation) takes place preferentially at the outer surfaces of the crystals, which have access to the oxygen gas atmosphere. — These cation vacancy processes can be studied directly with the help of a solid state galvanic cell, in which the evolving oxygen gas from the crystal surface is coulometrically titrated. The experimental data are in accordance with theoretical calculations which have been performed in order to understand under which conditions point defect concentration relaxation has an influence on chemical interdiffusion processes.