Tuning Sodium Ion Conductivity in the Layered Honeycomb Oxide Na3-xSn2-xSbx,NaO6

A series of compounds with the composition Na3-xSn2-xSbxNaO6 (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 alpha-NaFeO2, with alternating Na-3 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 Sbs for Sn-4 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 x 10(-3) S/cm at 500 degrees C with suboptimal sample densities). This material may be of interest as a solid Na ion electrolyte.