Understanding Na mobility in NASICON materials:: A rietveld, 23Na and 31P MAS NMR, and impedance study

The structures and electrical properties of four NASICON compositions, Na1.4M1.6In0.4(PO4)(3) (M = Ti, Sn, I-If 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 P-31 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. Na-23 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 degrees C, indicating high Na mobility at room temperature. Impedance data obtained on pressed sintered pellets over the range 25-300 degrees 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.