Tuning Sodium Ion Conductivity in the Layered Honeycomb Oxide Na_3-xSn_2-xSb_xNaO₆

A series of compounds with the composition Na_3-xSn_2-xSb_xNaO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, and 1.0) has been prepared by solid-state reaction and characterized by powder X-ray diffraction, neutron diffraction (for x = 0.0), and impedance spectroscopy. The compounds have a layered structure derived from that of α-NaFeO₂, with alternating Na₃ planes and NaSn₂O₆ slabs with honeycomb in-plane ordering. The structure of the parent compound, Na₂SnO₃, has been determined as a two-layer honeycomb in monoclinic space group C2/c. Due to charge neutrality requirements, the substitution of Sb⁵⁺ for Sn⁴⁺ creates sodium site vacancies that facilitate high sodium ion mobility. A decrease in layer stacking disorder is also observed. The conductivity increases linearly with x and has a maximum at x = 0.8 (1.43 × 10^-3 S/cm at 500 °C with suboptimal sample densities). This material may be of interest as a solid Na ion electrolyte. (Figure Presented)