The Na solid-state MAS NMR spectrum of NaNb7O18 revealed two exceptionally distinct environments, namely, a sharp resonance centered at -8.8 ppm and a broad resonance centered at -48 ppm (Figure ). Fitting the isotopic shift and quadrupolar parameters of these resonances yielded iso = -7.9(5) ppm and CQ ≤ 0.60 MHz for the narrow site and iso = -15(1) ppm, CQ = 3.78(3) MHz, and Q = 0.42(3) for the broad site. Similar lineshapes, with very different intensities, were observed in the related compound NaNb13O33, where it was suggested that the broad resonance comes from square planar Na1 with NaO4 coordination and the sharp resonance could be perovskite A-site like sodium. The crystal chemistry of NaNb13O33 is such that all sodium could be accommodated in NaO4 sites, and yet there was evidence for these tunnel-interstitial defects. Meanwhile, the stoichiometry and structure of NaNb7O18 require a high tunnel Na content. Small fractional occupancies in NaNb13O33 would require large supercells to set up meaningful calculations of NMR parameters, but these are readily modeled than the expected CT intensity if CQ is small. In the case of the disparity in the quadrupolar coupling constants between the square planar and cuboctahedral sodium sites in NaNb7O18, spectral simulations show that ST intensity will be observed in the centerband of the low-CQ cuboctahedral site and CT intensity will be spread into sidebands for the high-CQ square planar Li7NaNb7O18) could be accessed by discharging the NaNb7O18 electrode down to 1.0 V vs.