Locally Modulated Structures of Fluorite-Related Y₂O₃–Nb₂O₅ Solid Solutions

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. [10] diffraction patterns showed a pair of diffuse spots in the [11] 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 (10) plane is described by compositional waves of Y³⁺/Nb⁵⁺ and O²⁻/vacancy and displacive waves of Y³⁺ + Nb⁵⁺ and O²⁻ along the [11] 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.