The migration of atoms within a solid is strongly influenced by the defects associated with non-stoichiometry.  These defect sites provide pathways for atoms and ions to migrate through the otherwise dense ensemble of atoms that form the crystals.  Oxygen sensors and solid state batteries  are two applications that rely on oxide vacancies.  One example is the CeO-based sensor in automotive exhaust systems.  At low partial pressures of O, the sensor allows the introduction of increased air to effect more thorough combustion.