Synthesis and Lithium-Ion Conductivity of LiSrB₂O₆F (B = Nb⁵⁺, Ta⁵⁺) with a Pyrochlore Structure

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This study investigated the conductivity of lithium ions in new pyrochlore materials. A two-step synthesis method, comprising a simple solid-state reaction followed by a high-pressure solid-state reaction was used to synthesize single-phase undoped LiSrB₂O₆F (where B = Nb⁵⁺ or Ta⁵⁺). The samples were doped with the lower valent cation, Zr⁴⁺, which replaced Nb⁵⁺ or Ta⁵⁺ at B-sites in LiSrB₂O₆F and similarly, A-site point defects were introduced by reducing the amount of Sr in the sample. The crystal structure was determined by Rietveld analysis of the X-ray synchrotron diffraction data. Ionic conductivities of the samples were evaluated using electrochemical impedance spectroscopy. Sr²⁺ deficient samples showed better conductivity (8.21 × 10⁻⁶ S cm⁻¹ for Nb-containing samples and 6.38 × 10⁻⁶ S cm⁻¹ for Ta-containing samples at 300°C) than that of Zr⁴⁺-doped samples (5.58 × 10⁻⁶ S cm⁻¹ for Nb-containing samples and 3.58 × 10⁻⁶ S cm⁻¹ for Ta-containing samples at 300°C).