If this quanUty is posiUve, there would exist a thermodynamic driving force for conUnuous transiUon of surface N vacancies to the bulk, eventually resulUng in loss of the nitride. As such, a stable material should exhibit surface N vacancies that remain stable and are not filled by bulk N. Note that this criteria is consistent with discarding facile bulk N diffusion as a necessary criteria for a mediator: formaUon of a bulk nitride is not necessary, since a material that has stable surface nitride vacancies will not meaningfully interact with the bulk nitride, if it exists.