EXTENDED DEFECT INTERGROWTHS IN ZR1-XTI1+XO4

The low-temperature, Zr/Ti ordered, form of zirconium titanate has been investigated using high-resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO4 to near Zr5Ti7O24. Electron diffraction reveals that compositions with Zr:Ti between 5:7 and 1:1 have incommensurate superstructures, and phases close to 1:1 are commensurate with an a-axis repeat 2 X that of the disordered structure. The a-doubling in the 1:1 phase corresponds to two Zr-rich layers alternated with two Ti-rich octahedral layers. The incommensurate compositions are composed of blocks of the 1:1 structure intercalated with blocks of the 3 X commensurate 5:7 superstructure. The incommensurate structure can be described based on a quasi-periodic insertion of (100) anti-phase boundaries along a. The boundaries are uniformly distributed in such a way as to produce incommensurate satellite reflections. The main driving force for the ordering transformation appears to be the reduction of the number of nearest-neighbor Zr and Ti within the cation layers. This reduction is accomplished by the formation of domain-like segregations of Zr and Ti within the layers, and by "layer reactions" which prevent the formation of Zr-Ti-Zr linkages between layers. The layer reactions assure that the distribution of (100) faults is well-mixed in three dimensions, thereby permitting incommensurate diffraction maxima to occur.