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Cotunnite‐type 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 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. Tne 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 ini‐tio calculations.
Haines et al. (Wed,) studied this question.