Chemical stability of Li-stuffed garnet-type Li_{5 + x}Ba _xLa_{3 - X}Ta₂O₁₂ (x = 0, 0.5, 1) in water: A comparative analysis with the Nb analogue

In this study, we report the synthesis, chemical stability, and Li ion conductivity of garnet-type Li_{5 + x}Ba_xLa _{3 - x}Ta₂O₁₂ (x = 0, 0.5, 1) in water and discuss the chemical stability in comparison to the corresponding Nb analogue. Solid-state (ceramic) synthesis method was used to prepare the investigated compounds which exhibited a cubic garnet-type structure when sintered at 950 C in air. The ionic conductivity increases with increasing Li and Ba content in Li_{5 + x}Ba_xLa_{3 - x}Ta₂O₁₂, and the x = 1 member shows the highest bulk conductivity of about 10 ^{- 4} Scm^{- 1} at room temperature and the lowest activation energy of 0.39 eV (25 C-125 C). The Ta-based Li-stuffed Li _{5 + x}Ba_xLa_{3 - x}Ta₂O₁₂ garnets show better chemical stability in water compared to the corresponding Nb analogue, as supported by thermo-gravimetric analysis (TGA), and variable temperature powder X-ray diffraction (VT-PXRD). The proton-exchange was found to decrease with increasing Li content in Li_{5 + x}Ba_xLa _{3 - x}Ta₂O₁₂ in water, similar to the trend in the Nb series, Li_{5 + x}Ba_xLa_{3 - x}Nb ₂O₁₂. The slightly improved chemical stability of the Ta garnets (x = 0) in moisture may be due to the lower electronegativity of Ta compared to Nb, resulting in greater covalency of the Ta-O bonds, making them more difficult to break.