Phases in the NaNbO₃-Nb₂O₅-WO₃ system : derivation of complex structures by twinning, fourling and anti-phase-boundary operations on the ReO₃-type structure

Abstract: The compounds of the title system are reviewed with emphasis on their crystal structures. The principles used in describing them include not only the well-known concept of crystallographic shear in ReOg and the concomitant notion of block structures but also new applications of the idea of chemical twinning of an ReO^-type structure on the unit cell level. Twinning parallel to X2Î0) of the ReOg-type is found in structures of or related to the tetragonal and hexagonal tungsten bronze type. Twinning parallel to (310) of an ReOg-type with anti-phase boundaries constitute structures related to inter alia the alfa - UgOg structure.