measured on the POWGEN diffractometer at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. The neutron sample consisted of 4.648 g of NaNb7O18 and was loaded into an 8-mm-diameter cylindrical vanadium can because vanadium is neutron transparent. The sample was measured at 20 K and 300 K with the POWGEN sample changer (PAC). Data were collected on POWGEN detector bank 2 with a center wavelength (c) of 1.500 Å and d-spacing range 0.5-12.5 Å at both temperatures and on detector bank 3 with a center wavelength of 2.665 Å and d-spacing range 1.1-22.2 Å at 300 K. Data collection times for c = 1.500 Å at 20 K, c = 1.500 Å at 300 K, and c = 2.665 Å at 300 K were 87, 57, and 5 mins, respectively. Rietveld refinements were performed in GSAS-II. The neutron wavevector Q is defined as Q = 2/d. Solid-state NMR experiments.