LOCALLY MODULATED STRUCTURES OF FLUORITE-RELATED Y2O3-NB2O5 SOLID-SOLUTIONS

Modulated structures formed in defect fluorite-type (Y2O3)(1-x)(Nb2O(5 ))(x) (0.21 less than or equal to x less than or equal to 0.27) solid solutions have been investigated by electron diffraction. [1 (1) over bar 0] diffraction patterns showed a pair of diffuse spots in the [11 (2) over bar] direction at both sides of the 1/21/21/2 reciprocal-latt ice point. The distance between the diffuse spots decreased from 0.064 to 0.045 Angstrom(-1) with increasing x from 0.21 to 0.27. High-resol ution electron microscopy images revealed the existence of small domai ns with an antiphase structure. The two-dimensional antiphase structur e projected on the (1 (1) over bar 0) plane is described by compositio nal waves of Y3+/Nb5+ and O2-/vacancy and displacive waves of Y3+ + Nb 5+ and O2- along the [11 (2) over bar] direction. The composition at c ation sites is expressed by a sinusoidal wave with occupation changes between Y and 0.58Y + 0.32Nb. The occupation probability of O at anion sites is also expressed by a sinusoidal wave with occupation changes between 0.806 and 0.904. Amplitudes of the displacive waves for anions and cations were determined to be about 0.3 and 0.02 Angstrom. respec tively.