1. heating the ground starting materials in the furnace to 800 ºC at 10 ºC×min -1 and holding for 16 h followed by heating to 1280 ºC at 3 ºC×min -1 and holding for 2.5 h (Nb18W8O69-I); 2. placing the starting materials into the furnace directly at 1280 ºC for 50 min (Nb18W8O69-II); 3. placing the starting materials into the furnace directly at 1280 ºC for 8 h (Nb18W8O69-III) (Supplementary Figure ). These three conditions were chosen to balance the competition between reactant instability and product stability at high temperature: after 24 h at 1200 ºC, mostly Nb2WO8 was found by powder X-ray diffraction (PXRD) but with some shear structure reflections. After 24 h at 1300 ºC, mostly Nb16W5O55 was found by PXRD, which is consistent with the loss of WO3 via sublimation. The stability range of 1265-1375 ºC for Nb18W8O69 from Roth and Waring 23 was based on an annealing time of 1-3 h in a sealed Pt tube and they always observed Nb16W5O55 as a secondary phase. The PXRD patterns and electron microscopy images of the three preparations of Nb18W8O69 are nearly identical (Figure , Supplementary Figure ), but with notable differences in some low angle reflections (Figure ). All three preparations of Nb18W8O69 exhibit particles with subhedral to euhedral columnar crystal habit and typical dimensions of several µm in diameter by ~10 µm in ). Rietveld refinement of the Nb18W8O69 structural model was performed with the X-ray diffraction data from Nb18W8O69-II (Figure ). It was immediately apparent that the peak shapes are almost purely Lorentzian. The atomic positions, isotropic atomic displacement parameters, and cation site fractions were refined in GSAS-II . The light atom (i.e. oxygen) parameters are not reliable from the laboratory X-ray data but some tentative conclusions can be drawn regarding niobium and tungsten occupancies (Supplementary Table ). If Nb 5+ and W 6+ were distributed randomly across all cation sites, the expected tungsten partial occupancy would be 4/13 » 0.31. However, the data suggest that tungsten prefers sites M2-4 at the center of the blocks while niobium is enriched at M5-8 along the crystallographic shear planes. When niobium and tungsten are allowed to mix on the tetrahedral M1 site during the Rietveld refinement (Supplementary Table ), the tungsten occupancy moves away from unity, in contrast to the assumption made by Roth and Wadsley. Cheetham and Allen observed a strong preference for Nb 5+ at the edges of the ( ´ 4)1 blocks in Nb14W3O44. Koçer et al. calculated the cation configurations in Nb14W3O44 and Nb16W5O55 with DFT and Boltzmann statistics. The computational and experimental studies are in agreement; electrostatic repulsion at the edge-shared octahedra along the shear planes leads to preferred cation ordering with the higher charged cation in the block center and the lower charged cation at the block edges. A similar trend was observed in the TiO2•Nb2O5 block structures; in this case, though, Nb 5+ is the higher charged species. Cheetham and Von Dreele 21,28 used neutrons to show that Nb 5+ prefers the central octahedron in ( ´ 3)¥ TiNb2O7-recently confirmed with X-rays for this compound and the Ta analogue . The same phenomenon has been reported in the orthorhombic polymorph of ( ´ 4)¥ Ti2Nb10O29 21,28 and in (3 ´ 4)2 TiNb24O62 13 .