The narrowing of band gaps at GBs represents a challenge in the development of solid electrolytes; an efficient ionic conductor must have poor electrical conductivity, which is achieved by having a wide band gap. Clearly, GBs with a very narrow band gap would have higher electrical conductivity which would encourage leakage current, reducing the efficiency of the device. For example, in the case where the narrowing of the band gap originates from states below the CBM, we would expect free electrons to segregate towards these lowerenergy states at the boundary, leading to leakage current being dominated by the higher density of electrons in the GB. This has been shown to contribute to the formation of Limetal dendrites or 'filaments' along GBs in Li 7 La 3 Zr 2 O 12 , 11 which would eventually lead to the failure of the device. None of the GBs studied here show severely narrowed gaps, and the localized nature of the GB states implies a high effective mass that would not be conducive to electrical conductivity. However, conduction of delocalized charge carriers within bandsas would be seen in a metal or a narrow-gap semiconductor -is not the only means by which electronic conduction takes place.