Ceramic samples of with , and 0.1 were prepared by conventional methods. The crystal structures and microstructures of the ceramic samples were studied using x‐ray diffraction, infrared spectra, and scanning electron microscopy. The results show that a solid solution of was formed when was not larger than 0.05 and a second phase appeared in the solid solution when was equal to 0.1. The sinterability of improved greatly with addition of yttrium from 80.8% theoretical density (T.D.) for the sample without yttrium to 94.6% T.D. for the sample with ; the grain size decreased significantly with the formation of the solid solution. The total lithium ion conductivity increased slightly, from for pure to for the sample with at 400°C and the corresponding conduction activation energy decreased from 0.99 to 0.92 eV. Based on the effective medium theory, the conductivity increase in the solid solution crystal was estimated to be 3% for compared with the pure crystal.