A MODULATION WAVE APPROACH TO UNDERSTANDING THE DISORDERED STRUCTURE OF CUBIC STABILIZED ZIRCONIAS (CSZS)

A computer model of the distribution of oxygen vacancies in a cubic stabilized zirconia (CSZ) has been directly synthesized by applying modulation waves to the fluorite-type average structure. Only waves having wave vectors defined by circles centered at the 1/2{111}* reciprocal positions and in planes normal to the {111}* reciprocal directions were used for the synthesis. The derived oxygen-vacancy distribution was then used in conjunction with a previously reported method of applying cation relaxations to obtain computed diffraction patterns for comparison with the observed X-ray data. When the quantity modulated was taken as the occupancy of individual oxygen sites, the calculated diffraction patterns were unsatisfactory. However when the modulations were applied to 1/2[111] pairs of sites in each of four different orientations within the cube surrounding each cation site, the calculated diffraction patterns showed good qualitative agreement with the form of the observed patterns. The results of the study suggest a mechanism for the formation of the complex disordered structure. It is suggested that vacancy pairs oriented along a given [111] direction produce a strain field that extends to large distances in the plane normal to that direction so that like-oriented vacancy pairs repel each other. In contrast, unlike-oriented vacancy pairs do not interact strongly. Rings of diffuse intensity of different radii and different degrees of diffuseness may be interpreted in terms of the amount of strain induced by such vacancy-pair defects and the concentration of defects demanded by the composition. (C) 1995 Academic Press, Inc.