sentences:  In 1936 he published a crucial paper “"On the mechanism of the formation of ionic crystals of higher order (double salts, spinels, silicates)"” a concept of counter-diffusion of cations, which contributed to the understanding of all diffusion controlled, solid state reactions.Over a twenty-year period he produced an important body of work relating to the bulk transport processes in oxides.Wagner and Schottky proposed the point defect-mediated mechanism of mass transport in solids, Wagner then extended the analysis to electronic defects.