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 st abilized zirconia (CSZ) has been directly synthesized by applying modu lation 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 wer e 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 compar ison with the observed X-ray data. When the quantity modulated was tak en as the occupancy of individual oxygen sites, the calculated diffrac tion patterns were unsatisfactory. However when the modulations were a pplied to 1/2[111] pairs of sites in each of four different orientatio ns within the cube surrounding each cation site, the calculated diffra ction patterns showed good qualitative agreement with the form of the observed patterns. The results of the study suggest a mechanism for th e 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 dire ction so that like-oriented vacancy pairs repel each other. In contras t, unlike-oriented vacancy pairs do not interact strongly. Rings of di ffuse intensity of different radii and different degrees of diffusenes s 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.