Understanding Na Mobility in NASICON Materials: A Rietveld, ²³Na and ³¹P MAS NMR, and Impedance Study

Abstract

The structures and electrical properties of four NASICON compositions, Na1.4M1.6In0.4(PO4)3 (M = Ti, Sn, Hf, Zr), have been determined and compared. Rietveld refinement of powder X-ray diffraction data confirmed the basic rhombohedral NASICON structure with random occupancy of the octahedral In/M sites, full occupancy of the Na(1) sites and partial occupancy of the Na(2) sites. For three compositions, M = Zr, Sn, and Hf, the 31P MAS NMR peak intensities of the four detected signals, attributed to four different phosphorus environments [P(OM)4-n(OIn)n (n = 0−3)], were close to the ratios expected for a random distribution of In/M. For M = Ti, some departures from statistical occupancy were apparent. 23Na MAS NMR data gave evidence for two Na+ positions at room temperature for M = Ti, Sn, attributable to occupation of Na(1) and Na(2) sites. For M = Hf, Zr, only a single signal could be resolved at room temperature, which splits into two signals on cooling to − 50 °C, indicating high Na mobility at room temperature. Impedance data obtained on pressed sintered pellets over the range 25−300 °C showed that bulk ionic conductivities increased and activation energies decreased in the sequence Ti, Sn, Hf, Zr. The geometry of the M1M2 bottleneck has been determined from structural data, and a direct correlation found between activation energy for ion conduction and the bottleneck size.