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−x La3−x Ce x Zr2O12. 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.