Internal reduction of (Mg,Ni) O: Effect of a NiO-Concentration gradient

Single crystals of (Mg, Ni) O with a Ni-concentration gradient from the outer surface to the interior have been reduced so that metallic nickel particles precipitate in the wüstite matrix. The internal reduction morphology is very different from the one observed in reduced homogeneous (Mg, Ni) O single crystals: pores develop and the precipitates grow along preferential crystallographic directions. The addition of iron ions to the inhomogeneous mixed oxide crystals suppresses the porosity. The observations are described and discussed in terms of point defects and transport kinetics.