Crystal chemistry and ionic conductivity of garnet-type solid-state electrolyte, Li5-xLa3(NbTa)O12-y

Crystal structures, microtopography, morphologies, elemental compositions, and ionic conductivity have been investigated for Li 5- x La 3 (Nb,Ta)O 12- y using X-ray diffraction (XRD), field-emission analytical scanning and transmission electron microscopies (S/TEM), and electrochemical impedance spectroscopy. Using Rietveld refinements with powder XRD patterns, we determined that the number of Li atoms in the formula is less than 5 and that Li 5- x La 3 (NbTa)O 12- y crystallizes in the cubic garnet structure with a space group Ia- 3 d . Sintering at varying temperatures (750–1000 °C) for 5 h in an ambient atmosphere produced distinct outcomes. Rietveld refinements disclosed that the sample sintered at 1000 °C (Li 3.43(2) La 3 Nb 1.07(2) Ta 0.93(2) O 12- y , a = 12.8361(7) Å, V = 2114.96(3) Å3) exhibited the highest ionic conductivity, while the 850 °C sample had the lowest conductivity, characterized by lower Li concentration and impurity phases (Li(Nb,Ta) 3 O 88 , Li 2 CO 3 ). Analyses, including XRD and electron microscopy, confirmed the 1000 °C sample as a relatively phase pure with enhanced Li content (Li/La = 1.2), larger grains (15 μm), and uniform crystallinity. The 1000 °C sample introduced additional partially filled Li3 (96 h ) sites, promoting Li migration, and enhancing ionic conductivity. The resulting XRD pattern at 1000 °C has been submitted to the Powder Diffraction File as a reference.