As equilibrium concentrations give an indication of the solubility of a specific dopant, out predicted value for Ga is several orders of magnitude lower than the other dopants, and doping with Ga is known to result in the formation of secondary phases. Moreover, it has been reported by Schwab et al. that 0.20 Ga per formula unit exceed the solubility limit of Ga in LLZO, and in their study, the amount of the secondary phases formed could not be detected by XRD. Furthermore, if we assume that all Ga added in this experiment result in the formation of LiGaO 2 , the resulting volume fraction is ∼ 3%, relatively close to the detection limit of regular XRDs. Therefore, the formation of relatively small amounts of secondary phases can easily be missed.