CRYSTAL-STRUCTURE AND EQUATION OF STATE OF COTUNNITE-TYPE ZIRCONIA

Cotunnite-tppe zirconia was studied by angular-dispersive X-ray diffraction in a diamond anvil cell after laser heating at 18 and 26.7 GPa. The structure, space group Pnam, Z = 4, was refined in situ at several different pressures on decompression and at ambient using the Rietveld method. The nine polyhedral Zr-O distances range from 2.10 to 2.56 Angstrom A at ambient pressure, which represents both an increase in the average and the minimum Zr-O distance relative to the monoclinic and orthorhombic lower-pressure forms. In addition, data obtained on an unheated sample indicate that the irreversible transition to the cotunnite phase began above 25 GPa and 70% conversion was achieved by 48.5 GPa. The compressibility of cotunnite-type zirconia was found to be slightly anisotropic and a Birch-Murnaghan equation-of-state fit of the p-V data yielded a bulk modulus of 332(8) GPa with a first derivative of 2.3(4), which is in good agreement with the value predicted by previous ab initio calculations.