Floating zone crystal growth, structure, and properties of a cubic Li_5.5La₃Nb_1.5Zr_0.5O₁₂ garnet-type lithium-ion conductor

As a promising candidate for solid-state electrolytes in Li-ion batteries, the garnet-type Li-ion conductor series Li_5+xLa₃Nb_2−xZr_xO₁₂ (LLNZO) (0 ≤ x ≤ 2) exhibits high ionic conductivity at room temperature. However, no previous single-crystal growth or characterization has been reported for LLNZO compositions 0 ≤ x ≤ 1. To obtain a complete understanding of the trend in the structure-property relationship in this class of materials, we used the floating zone (FZ) method to grow a single crystal of Li_5.5La₃Nb_1.5Zr_0.5O₁₂ that was 4 mm in diameter and 10 mm in length. Using Laue neutron single-crystal diffraction, two distinct Li sites were observed: tetrahedral 24d and octahedral 96h sites. The maximum entropy method (MEM) based on neutron single-crystal diffraction data was used to map Li nuclear density and estimate that the bottleneck of Li transport exists between neighboring tetrahedral and octahedral sites, and that Li is delocalized between split octahedral sites. Room-temperature Li-ion conductivity in Li_5.5La₃Nb_1.5Zr_0.5O₁₂ measured with electrochemical impedance spectroscopy (EIS) was 1.37 × 10⁻⁴ S cm⁻¹. The Li migration activation energy was estimated to be 0.50 eV from EIS and 0.47 eV from dielectric relaxation measurements. The Li-ion jump attempt rate was estimated to be 1.47 × 10¹² Hz while the time scale of successful migration is 10⁻⁷ to 10⁻⁶ s.