Effect of lithium isotopes on the phase transition in NASICON-type lithium-ion conductor LiZr2(PO4)3

Polycrystalline NASICON-type Li-ion conductor LiZr2(PO4)3 (LZPO) with different ratios of Li isotopes, namely 6LiZr2(PO4)3 (6-LZPO), 7LiZr2(PO4)3 (7-LZPO), and LZPO with the natural Li isotope ratio (n-LZPO), has been synthesized by a conventional solid state reaction. The phase transformation as a function of temperature between the low-temperature triclinic phase, which exhibits lower Li-ion conductivity, and high-temperature rhombohedral phase, which exhibits higher Li-ion conductivity, has been evaluated by powder X-ray diffraction (XRD), electron diffraction, and differential scanning calorimetry (DSC) measurements. According to XRD and DSC measurements, the phase transition temperature decreases in the order 6-LZPO > n-LZPO ≈ 7-LZPO. The dependence of phase transition temperature on the Li isotope implies that Li ions have a strong effect on phase stability. The phase transition is primarily related to the change in configuration entropy of Li ions in LZPO.