Reagents and synthesis. NaNb7O18 was synthesized in multi-gram batches from Na2CO3 (anhydrous after drying for 4 h at 250 C, 99.95%, Sigma Aldrich) and T-Nb2O5 (99.85%, D50 = 450 nm, CBMM). A 1:7 mol ratio of Na:Nb precursors were ground with an agate mortar and pestle and pressed into a pellet at 300 MPa with a 13-mm-diameter stainless-steel die. The pellet was heated in a platinum crucible at 5 C min -1 to 1175 °C for 18 hours before quenching by removing it from the furnace and placing it on a steel plate that served as a heatsink. Three heat-quench cycles with intermediate regrinding and re-pelleting were required to minimize the quantity of impurity. Rapid quenching was also found to be very important, consistent with the metastable nature of the phase. Laboratory powder X-ray diffraction, neutron diffraction, and Na NMR all revealed a small impurity of bronze-like Na13Nb35O94 (~3-4%).