Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li<sub>7−</sub><i><sub>x</sub></i>La<sub>3</sub>Zr<sub>2−</sub><i><sub>x</sub></i>Ta<i><sub>x</sub></i>O<sub>12</sub> (0 ≤ <i>x</i> ≤ 0.6)

We investigated the phase formation and the conductive properties of the Al-free Li7−xLa3Zr2−xTaxO12 (0 ≤ x ≤ 0.6) samples. The X-ray diffraction patterns of the Li7La3Zr2O12 (x = 0) and Li6.4La3Zr1.4Ta0.6O12 (x = 0.6) samples were assigned to be single phases of tetragonal (space group: I41/acd) and cubic (space group: Ia-3d) structures, respectively. On the other hand, the intermediate compositional samples of Li7−xLa3Zr2−xTaxO12 (0.2 ≤ x ≤ 0.5) showed a coexistence of both the tetragonal and cubic phases. To investigate the conductive property of the prepared samples, the Li-ion conductivity was measured in a temperature range from 253 to 313 K by AC impedance method. All of the Al-free Li7−xLa3Zr2−xTaxO12 (0 ≤ x ≤ 0.6) samples exhibited relatively high conductivity of ∼10−4 S cm−1 at room temperature, and the Li6.5La3Zr1.5Ta0.5O12 (x = 0.5) sample showed the highest Li-ion conductivity of 8.4 × 10−4 S cm−1 at room temperature. In order to clarify the relationship between the Li-ion conductivity and the Li-ion arrangement, the crystal structure analysis of Li7−xLa3Zr2−xTaxO12 (0 ≤ x ≤ 0.6) was performed by Rietveld analysis using powder X-ray diffraction data. The Li(2) atom at 96h site was gradually shifted together with increasing Ta-content from x = 0.2 to 0.5 resulting the shorter Li–Li distance in the loop structure of the cubic garnet-type framework structure.