THE POLYMORPHS OF ZIRCONIA - PHASE ABUNDANCE AND CRYSTAL-STRUCTURE BY RIETVELD ANALYSIS OF NEUTRON AND X-RAY-DIFFRACTION DATA

Rietveld analysis of neutron and X-ray powder diffraction data has been used to obtain the crystallinity and relative abundances of cubic (22 at% Y), tetragonal (5.8 at% Y), and monoclinic zirconia in synthetic binary and ternary mixtures of the pure phases. The stabilized cubic and tetragonal forms are shown to be 94 (2) and 96 (2) % crystalline, respectively, but no amorphous material was detected in the chemically pure monoclinic phase. In both stabilized polymorphs, the yttrium atoms randomly substitute into the zirconium site, with a charge-compensating proportion of vacancies on the oxygen atom site. In this near worst-case situation of very similar cubic and tetragonal unit cell dimensions, the phase abundances determined by Rietveld analysis of neutron data are very accurate and superior to those determined by integrated-intensity methods of analysis. For X-ray data, the accuracy is diminished by the presence of extinction, and by uncertainty in the definition of the peak shape of the cubic phase due to the absence of intense high-angle reflections and the resultant dependence on strongly overlapping low-angle data.