Unique Growth Manner of Li₅La₃Ta₂O₁₂ Crystals from Lithium Hydroxide Flux at Low Temperature

Garnet-type Li_5+xLn₃M^IV_2-yM^V_yO_12+z (x, y = 0-2, z = 0-1; Ln = La, Pr, Nd; M = Ta, Zr, Nb) compounds are promising Li-ion conducting solid electrolytes, but their growth manner is still unclear. Herein, the analysis of the low-temperature growth of idiomorphic Li₅La₃Ta₂O₁₂ single crystals as a function of holding temperature and time, cooling rate, flux type, and solute concentration revealed a unique growth manner. Li₅La₃Ta₂O₁₂ crystals were grown at 500 °C from LiOH flux and transformed into Li₇La₃Ta₂O₁₃ at 700 °C. The pseudo-perovskite-type LiLa₂TaO₆ phase, initially formed during the holding at 500 °C, was efficiently transformed into the Li₅La₃Ta₂O₁₂ phase with increasing holding time. The growth of Li₅La₃Ta₂O₁₂ single crystals was independent of the cooling rate but was affected by the kind of flux and solute concentration. A low solute concentration (1 or 5 mol %) was the key to obtain well-dispersed and idiomorphic single crystals. The optimum growth conditions involved a holding temperature of 500 °C, a solute concentration of 1 or 5 mol %, and a holding time of 10 h. These findings indicate that formation and growth of Li₅La₃Ta₂O₁₂ single crystals are not only controlled by a general flux growth process but also involve chemical reactions between solutes and LiOH flux. Finally, high-resolution transmission electron microscopy image and selected area diffraction pattern highlighted products with high crystallinity and well-developed {110} and {211} facets.