The garnet-type ceramic electrolyte of nominal composition Li7La3Zr2O12 can exist in the tetragonal and cubic form. This article investigates the tetragonal to cubic phase transition based on super-valent cation substitution on the 24c site typically occupied by La (3+) in the garnet structure. Ce (4+) was selected as the super-valent cation represented as x in Li7-xLa3-xCexZr2O12. The doping study showed that cubic LLZO was stabilized for Ce >= 0.2. These data agree with most literature reports suggesting that the creation of Li vacancies, while maintaining oxygen stoichiometry, is necessary to stabilize cubic LLZO. Moreover, this work suggests a critical Li vacancy concentration (0.12-0.4 mol per mole of Li7La3Zr2O12) is necessary to stabilize cubic LLZO. While the addition of Ce stabilized cubic LLZO, the total ionic conductivity (0.014 mS cm(-1)) was considerably lower compared to Al and Ta doped LLZO (0.4-0.9 mS cm(-1), respectively). The lower conductivity is likely due to CeO2 precipitation at grain boundaries. (C) 2012 Elsevier B.V. All rights reserved.