Structural change-conductivity relationship in Na4–xZr2–xNbxSi3O12 solid electrolytes over a wide temperature range

Samples of Na 4–x Zr 2-x Nb x Si 3 O 12 ( x = 0–0.4) system are synthesized, phase analysis is performed, and the conductivity is measured. Samples with x = 0–0.25 are single-phase with a NASICON-type trigonal symmetry, space group R-3c. Na 3.85 Zr 1.85 Nb 0.15 Si 3 O 12 solid electrolyte exhibits the highest sodium-cation conductivity (1.7 × 10 −2 S × cm −1 at 300°, 10 −5 S × cm −1 at 25 °C). High-temperature X-ray studies in the range of 25 to 727 °C were performed for this composition. Full-profile analysis was carried out for all temperatures to refine the structural parameters. Based on the experimental data, two elementary channels open for sodium cation migration were found using the method of tilings; the channels form a continuous three-dimensional network of migration paths. It is shown that the break of the lgσ–1/T plot at 480 °C is due to a sharp decrease in the strength of the Na–O bond in NaO 6 octahedra and the consequent increase in the mobility of sodium cations in 6b (000) sites.