On processing structure-conductivity relations in NASICON-type LiSn₂(PO₄)₃

The present investigation focuses on understanding the structure-electric conductivity correlation in NASICON-type LiSn₂(PO₄)₃ (LSP) powders prepared via solid-state reaction method. LSP powders synthesized at different temperatures were characterized for their structural and electrical properties using lab source powder X-ray diffraction (XRD), high-resolution synchrotron X-ray diffraction (SXRD) and complex impedance spectroscopy. LSP powders prepared in 900-1000°C temperature crystallize in triclinic structure (space group, P1⎯⎯⎯) along with the small amount of SnO₂ (P42/mnm) impurity phase. Samples prepared at temperatures in 1050-1250°C range showed a mixed rhombohedral (R3⎯⎯⎯c) and triclinic structure with the fraction of the triclinic phase decreasing with an increase in calcination temperature. On further increase in the calcination temperature to 1300°C, LSP transformed to the rhombohedral structure. Moreover, temperature-dependent SXRD confirmed that the LSP powder exhibits a martensitic behaviour, where a pure triclinic structure transforms into a pure rhombohedral phase at 170°C and retains a partial rhombohedral phase on cooling back to room temperature. The highest value of conductivity was found to be ~1.06 × 10^-6 Scm^-1 for the LSP powder with triclinic structure calcined at 900°C, with an associated activation energy of ~0.24 eV. Rhombohedral LSP calcined at 1300°C exhibits the lowest conductivity and highest activation energy at room temperature ~1.12 × 10^-8 Scm^-1 and ~0.39 eV, respectively. This decrease in conductivity for the supposedly high-conducting rhombohedral phase is attributed to the drastic increase in the fraction of the SnO₂ impurity phase, as confirmed by the XRD analysis. (author)