Phase stability of the Garnet-structured lithium ion conductor Li7La3Zr2O12 (LIZ) was investigated using high-temperature X-ray diffraction (HT-XRD). The tetragonal LLZ containing CO2 transformed to the cubic phase around 150 degrees C. However, once the tetragonal LIZ heated up to 1000 degrees C, the cubic phase transformed back to the tetragonal phase again, because of the CO2 loss during the heating process. After the CO2 loss, the phase transformation temperature shifted to 650 degrees C. The HT-XRD study for Al-doping process was also performed. The addition of gamma-Al2O3 to the tetragonal LIZ eliminated the formation of impurity phase such as La2Zr2O7 and La2O3, resulting in the formation of a pure cubic phase. A comparison of the HT-XRD results suggested that the Li loss is necessary to stabilize the cubic phase with Al. (C) 2013 Elsevier B.V. All rights reserved.