The low-angle diffraction region is significant in cs phases because the long d-spacing inter-block arrangement differentiates various cs phases with nearly identical short-range structure motifs. The unindexed peaks in Figure , that are increasingly pronounced from Nb18W8O69-I to III, do not correspond to expected compounds from the phase diagram such as Nb16W5O55, Nb2WO8, Nb14W3O44, Nb12WO33, H-Nb2O5, or WO3. Another hypothesis is that these peaks relate to defects in Nb18W8O69. Wadsley defects-coherent intergrowths of different, usually adjacent, cs phasesare characteristic defects in niobium tungsten oxide shear phases. Allpress and Roth studied Nb18W8O69 with electron microscopy and observed both ( ´ 5) blocks of octahedra with the nominal composition Nb16W5O55 and ( ´ 6) blocks of octahedra with the theoretical composition Nb20W11O83. We postulate that the unidentified peaks observed here may be related to extended defect regions within Nb18W8O69.