For structures with solely Na1 vacancies (structure A) Na exchange between full and empty layers is sites in Al-rich environments, are crucial for ionic transport. This is further supported by the 23 Na- Na EXSY MAS NMR spectra (Figure ), where the strongest exchange with significant intensity is observed between Na2 and Na1 sites, both in mixed Al/Zn environments. This is completely consistent with the results from MD simulations of structure C (Figure , e). To summarize, the aliovalent substitution of Al 3+ for Zn 2+ (x > 0.25) leads to the formation of preferably Na1 vacancies (also resulting in Na2 sites in Al-rich environments) but requires the formation of additional Na2 vacancies for charge compensation.