A lithium ion conductor with the garnet-type structure, Li7La3Zr2O12 (LLZ), showing a high Li+ conductivity of 0.49 mS cm(-1) at 25 C, is synthesized via milling (500 rpm, 12 h) and following sintering at 1125 C for 20 h. The improved Li+ conductivity is attributed to the decrease of the particle size, the uniform distribution of particles before sintering, the high concentration of Li+ in the octahedral site (96h), and low occupancies of Li+ in the tetrahedral site (24d). Also, it is found that the cubic phase of LLZ is stabilized by a contamination of Al3+ ion into the LLZ lattice. The structural position of Al3+ is verified to be in the tetrahedral site (24d) by analyzing SEM images, MAS NMR, Pawley refinement using X-ray powder diffraction, and structural refinement using neutron powder diffraction data. (c) 2014 Elsevier B.V. All rights reserved.