Locally Modulated Structures of Fluorite-Related Y₂O₃–Nb₂O₅Solid Solutions

Abstract

Modulated structures formed in defect fluorite-type (Y2O3)1 − x(Nb2O5)x (0.21 ≤ x ≤ 0.27) solid solutions have been investigated by electron diffraction. [1{\bar 1}0] diffraction patterns showed a pair of diffuse spots in the [11{\bar 2}] direction at both sides of the ½½½ reciprocal-lattice point. The distance between the diffuse spots decreased from 0.064 to 0.045 Å−1 with increasing x from 0.21 to 0.27. High-resolution electron microscopy images revealed the existence of small domains with an antiphase structure. The two-dimensional antiphase structure projected on the (1{\bar 1}0) plane is described by compositional waves of Y3+/Nb5+ and O2−/vacancy and displacive waves of Y3+ + Nb5+ and O2− along the [11{\bar 2}] direction. The composition at cation sites is expressed by a sinusoidal wave with occupation changes between Y and 0.58Y + 0.42Nb. The occupational 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 Å, respectively.