The Systematics of Block-Structure Shift Lattices

The binary and ternary niobium oxides structurally related to H—Nb2O5 have structures made up of columns of ReO3-type linked by boundary crystallographic shear planes. They are known as block structures in view of their appearance in projection. In this paper, the structures have been treated as two-dimensional shift lattices in which the ReO3-like columns are regarded as lattice fragments. The paper shows how the shift-lattice parameters may be derived and these are used to illustrate how the structures can be described in a systematic fashion and how hypothetical or new structures can also be incorporated into the framework of known block structures. It is also shown that many Wadsley defects occurring in block structures can be explained as simple manifestations of the operation of shift-lattice principles and in such instances the term ‘defects’ is inappropriate. The paper concludes with a discussion of the way in which the shift-lattice approach avoids difficulties associated with semicommensurate and incommensurate diffraction patterns which can occur in block-structure systems.