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

Garnet-type Li5+xLn3MIV2–yMVyO12+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...