Sodium mobility in the NASICON series Na1+xZr2-xInx(PO4)3

Rhombohedral Na1+xZr2-xInx(PO4)(3) (x = 0, 0.2, 0.4, 0.8, 1.0, 1.2, 1.6, and 1.8) NASICON materials have been studied by XRPD, variable-temperature NPD, (31)Ip and variable-temperature Na-23 MAS NMR, and impedance spectroscopies. Relative P-31 MAS NMR peak intensities of the five detected signals, attributed to the environments [P(OZr)(4-n)(OIn)(n) (n = 0-4)], are close to those expected for a random distribution of octahedral cations. This local probe allows us to rule out the existence of segregated metal-rich nanoregions. Combined NPD and XRPD Rietveld studies showed that the occupation of M2 sites by the extra Na+ cations produces a slight distortion of the structure. Low temperatures freeze Na mobilities and permit the assignment of the resulting NMR bands to Na+ in the M1 and M2 sites. The mobility of Na at room temperature increases with the Na content. For samples with x < 0.8, Na ions are relatively localized; however, for x greater than or equal to 0.8, Na mobility increases, yielding a unique signal in the Na-23 NMR spectra. The increase of Na mobility causes spatial disorder at the M1 site as deduced from the variable-temperature NPD study. At low temperatures, the activation energies do not change significantly. So, the observed increase in bulk conductivity by 2 orders of magnitude in the low-temperature regime is ascribed to an increase of mobile carrier, Na+, concentration. However, at higher temperatures a new regime has been identified by NPD, Na-23 NIAS NMR, and Arrhenius plots of f(max) data in Z " spectra and is associated with more extended sodium motion. The onset temperature for the curvature in the Z " f(max) Arrhenius plots, when reaching the high-temperature regime, depends on sodium content.