Abstract
A thermodynamic model taking account of cation vacancies has been formulated for oxide solutions of type ( A, B) O and A(A, B) 2 O 4. The chemical potentials of the cation species are found to be functions of the vacancy concentration and of the free energy of formation of vacancies. A form has been chosen for the excess free energy of the solid solution which includes cation-cation and cation-cation vacancy interactions. The predicted type of dependence of the cation vacancy concentration on the oxygen activity has been compared with the observed dependence for (Fe, Mn) O. According to the theory a composition gradient will be set up when the oxide solid solution is placed in an oxygen activity gradient. Assuming the same diffusion rate for all cation species, composition curves have been calculated for (Fe, Mn) O and (Fe, Ni) O growing on alloys and for an solated crystal of Fe (Fe, Co) 2 O 4 in an oxygen activity gradient.
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