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With high-resolution electron microscopy new crystal structures were found in the binary system of Nb205-WO 3 oxides. The compounds are proposed to have the mole ratios 4Nb205. 22WO 3 and 4Nb205. 50WO 3. The crystal structures are ascribed to intimate intergrowths of two known structures, the ReO 3 type and tetragonal tungsten bronze type. The latter grows coherently in the host lattice of the former, forming regularly arranged slabs. The coherent intergrowth of the tetragonal tungsten bronze structures is described as the 'rotation operation' on the ReO3-type structure. IntroductionCompounds in the binary system of Nb205-WO3 oxides have been intensively studied. According to the studies on the phase diagram of this system by Roth & Wadsley (1965), the compounds are grouped into three different structure types: the 'block structures' occurring in the composition range of O/M (oxygen/metal ratio) = 2.50-2-66, the 'Magn~li phases' in the WO 3-rich region of O/M = 2.90-3.00, and in the range of O/M = 2.66-2.90 the tetragonal tungsten bronze-type structure (hereinafter abbreviated TTB type). For the TTB type, two ordered structures 4Nb205. 9WO 3 and 2Nb205. 7WO 3 were found. The block structures and the Magn61i phases having the ReO 3 type as a basic structure contain the crystallographic shear structures as a common structural component where the individual metal-oxygen octahedra are linked with their neighboring octahedra by edge sharing instead of corner sharing in the basic structure of the ReO 3 type. In the Magn61i phases the crystallographic shear planes are arranged one-dimensionally, forming slabs of the ReO 3 type, while in the block structures the crystallographic shear planes are arranged two-dimensionally, forming columns of the ReO 3 type. A slight change in composition results in rearrangement of the crystallographic shear planes and thus a slightly different structure from one compound to another, so that a homologous series of the compounds is formed (Wadsley & Andersson, 1970).The structure of 2Nb2Os.7WO3 was determined from the study of the high-resolution electron micrographs of crystals having a nominal composition of 19Nb2Os.63WO~ (lijima & Allpress, 1974a) and was found to result from an intergrowth of the two different structures of ReO 3 type and the TTB type. It was proposed that since the small domains of the ReO3-type structure are intergrown coherently with the TTB type in 2Nb205. 7WO 3, a transformation of this structure from the ReO 3 type to the TI'B type, which occurs in solid-state reactions, would be conceivable by a periodic application of the rotation of the groups of the corner-shared octahedra to the ReO 3 type. Direct evidence of the rotation which causes formation of single elements of the TTB-type structure in some regions of the ReO 3 type has been demonstrated by Iijima & Allpress (1974b). This was observed often in disordered regions of crystals having a nominal composition of 19Nb2Os.63WO 3. More general considerations on structural relations between the ReO 3 type and ...