Prediction of high temperature demixing in oxides in an electric field or an oxygen potential gradient

During exposure to an oxygen potential gradient or an electric field at high temperatures, semi-conducting and initially homogeneous mixed ceramic oxides of the rock-salt type show demixing of the cations. The main assumption made in previous theoretical treatments of demixing is that the off-diagonal phenomenological transport coefficients can be ignored. In this paper, off-diagonal coefficients are calculated by making use of the exact sum-rule expression relating the phenomenological transport coefficients. The demixing analysis is then greatly simplified and made exact. The analysis is verified with Monte Carlo computer simulation. Results are presented for steady state composition profiles in oxygen potential gradients and electric fields. Unusual steady state demixing composition profiles are possible in the quaternary oxide (A,B,C)O. Demixing profiles in (Co,Mg)O (oxygen potential gradient) and (Co,Ni)O (electric field) are analysed.