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How can ion-exchange process occur in nanocrystals without the size and shape changing and why is the ion transport much faster than in classical interdiffusion processes in macrocrystalline solids? We have investigated these processes at the molecular level by means of high-resolution and analytical electron microscopy in temperature-dependent kinetic experiments for several model reactions. The results clearly show a diffusion process that proceeds exclusively through the interstitial lattice positions with a subsequent "kick out" to remove individual ions from lattice sites without the formation of vacancies. This mechanism has not been observed in nanocrystalline systems before. [ABSTRACT FROM AUTHOR]