Neutron diffraction data were collected at 20 K and at 300 K (Figure and Supplementary Figure ). At 20 K, atomic motion is suppressed and nuclear density is more localized, which Following the naming scheme used by Woodward and Beanland for AgNb7O18, we have named these sites Na2, Na3, and Na4, respectively. As suggested by the Fourier difference maps, atoms Na2 and Na3 were added at the approximate centers of the A-site cavities and then the sodium occupancies of Na1-3 were freely refined. The unconstrained composition refined to Na1.054±0.049Nb7O18, which is within about one standard deviation of the expected stoichiometry NaNb7O18. As the refinement proceeded, the composition was fixed at the expected value and one isotropic displacement parameter was refined for each atomic species. Attempts to add and refine the occupancy of Na4, even within the above constraints, were unstable and suggest that the occupancy of these corner tunnel sites is very close to zero in NaNb7O18. The final 20 K structure is available as Supporting Information and CSD 2406075. Refinement of the structure at 300 K, starting from the 20 K model, yielded very minor differences other than a 5-7x increase in thermal displacement factors (Supplementary Figure , CIF in Supporting Information). At both temperatures, the Na1 site refined to an occupancy of 0.87-0.88 corresponding to a crystal chemical formula of [Na sq.pl. ]0.44[Na cuboct. ]0.56Nb7O18 with an average A-site occupancy of 16%.