Abstract
Li x La(1-x)/3Nb1-y Ta y O3 (LLNT) is a solid electrolyte with a double perovskite-type structure. It was reported that the highest ion conductivity of LLNT was 7.78 × 10-5 S/cm at x = 0.1, y = 0.2 [1,2]. Therefore, in this study, we investigated the preparation conditions of the feed and the growth conditions of Li0.1La0.3Nb0.8Ta0.2O3 single crystals by the traveling solvent floating zone (TSFZ) method.Li2CO3, La2O3, Nb2O5, and Ta2O5 were used as starting materials, weighed at Li x La(1-x)/3Nb1-y Ta y O3 (x = 0.1, y = 0.2) stoichiometric composition, and mixed with ethanol. The mixed powder was calcined at 800 °C for 2 hours in air. The calcined powder was grounded and calcined again at 1200°C for 12 hours in air. The calcined powder was then shaped into a cylindrical shape by rubber pressing and sintered at 1350°C for 5 hours in air. Solvent of 5 mol% of La(Nb,Ta)3O9-poor, La(Nb0.5Ta0.5)O4-poor or La(Nb0.8Ta0.2)O4-poor composition relative to the stoichiometric composition of LLNT feeds was prepared using the same procedure as the feed rod preparation. The optical floating zone machine was used for crystal growth. The growth conditions were the growth rate of 2 or 5 mm/h, the growth atmosphere of Ar or N2. The grown crystals were characterized using X-ray diffraction, electron probe micro-analyzer (EPMA), and impedance measurements.As a result of crystal growth of LLNT by the floating zone (FZ) method, the La(Nb,Ta)3O9 phase was deposited as an inclusion in the initial growth region, even though Li evaporation from the molten zone was not observed. This result indicates that LLNT is an incongruently melting compound. Next, the TSFZ method, FZ method using solvents, was applied to the growth of LLNT crystals. La(Nb,Ta)O4 inclusion phase deposited in the initial growth part of the grown crystal when using solvent of 5 mol% La(Nb,Ta)3O9-poor composition relative to the stoichiometric LLNT. In the case of using a 5 mol% La(Nb0.8Ta0.2)O4-poor solvent composition, crystals with a length of 12 mm and a diameter of 5 mm were obtained as shown in Fig. 1(a). La(Nb,Ta)O4 inclusion phase was not detected in the grown crystal. The chemical composition of the LLNT phase was determined to be y = 0.14-0.25 by quantitative analysis using EPMA as shown in Fig. 1(b). Therefore, inclusion-free LLNT crystals were obtained using a solvent with 5 mol% La(Nb0.8Ta0.2)O4-poor composition relative to the stoichiometric LLNT composition. Moreover, the molten zone was stable in Ar gas or N2 gas flow at growth rate of 5 mm/h. The ionic conductivity of the grown crystal was σ= 2.0 ×10-5 S/cm . The low conductivity is due to cracks, and inhomogeneous in the grown LLNT crystal. Optimization of the solvent composition is required for LLNT crystals with homogeneous and high ionic conductivity.References.[1]Y. Maruyama, et. al., J. Ceram. Soc. Jpn. 128, 761-765 (2020). [2]Y. Maruyama, et. al., J. Ceram. Soc. Jpn. 129, 535-539 (2021). Figure 1
Published Version
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