LOCALLY MODULATED STRUCTURES OF THE Y₂O₃-Nb₂O₅ SOLID SOLUTIONS. R.Miida, *F.Satoh, *M.Tanaka,**H.Naito and **H.Arashi Sci. Univ. of Tokyo Suwa Coll. Chino Nagano Japan, *Res. Inst. Sci. Meas., **Fac. of Eng. Tohoku Univ. Sendai Japan

Modulated structures formed in the defect fluorite type (Y₂O₃)_1-x(Nb₂O₅)_x (0.21%X%0.27) solid solutions have been investigated by electron microscopy. The [1 1 0] diffraction pattern showed a pair of diffuse spots in the [1 1 2] direction at both sides of the reciprocal lattice point. The distance between the diffuse spots decreased from 0.064Å^-l to 0.045Å^-1 with increasing x from 0.21 to 0.27. The HREM images revealed the existence of small domains with an antiphase structure (APS). The twodimensional APS (x=0.21) projected on the (1 1 0) plane is described by compositional waves of Y/Nb and O/vac and displacement waves along the [1 1 2] direction, as shown in Fig.1. The composition at cation-sites is expressed by a sinusoidal wave, whose amplitude changes between Y and 0.58Y+0.42Nb. The occupation probability of O at anion-sites is expressed to change between 0.806 and 0.904 in the form of a sinusoidal function. Amplitudes of the displacement waves for anions and cations were estimated to be about 0.3Å and 0.02Å, respectively.

Fig.1 Two-dimensional APS model APB = antiphase boundary