Angular-dispersive x-ray in situ powder-diffraction experiments have been performed on pure zirconia, Zr02, at room temperature under high pressure up to 50 GPa. Under increasing pressure four phases were successively encountered: baddeleyite (monoclinic, P21/c) from normal pressure up to about 10 GPa, orthorhombic-1 (Pbca) to 25 GPa, orthorhombic-11 to 42 GPa, and orthorhombic-111 above 42 GPa. The unit-cell parameters and the volume have been determined as a function ofpressure. The bulk moduli of the two lower pressure phases have been calculated using Birch's equation of state. The bulk modulus of baddeleyite, 95 GPa, is much lower than expected from bu1k modulus-volume systematics, 195 GPa, while for the orthorhombic-1 phase, the experimental and calculated values are almost identical. A generalized P-T diagram for Zr02, including an orthorhombic-IV phase, is proposed and discussed. The phase transition to orthorhombic-11 and orthorhombic-111 phases can be described by a simple rotation of the unit cell of the orthorhombic-1 phase about either the b axis to form the orthorhombic-11 phase or a axis to form the orthorhombic-111 phase. All high-pressure cells (orthorhombic-1, -11, and -111) have eight formula units (Z =8). The orthorhombic-11 phase was found not to have the cotunnite PbC12-type structure which was proposed previously. There is no longer any examp1e of a compound which transforms to such a cotunnite-type structure under high pressure. The behavior of zirconia and hafnia under high pressure is different although they have very close chemical properties at ambient pressure and identical structures in the two lower-pressure phases.

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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, 2 = 4, was refined in situ at several different pressures on decompression and at ambient using the Rietveld method. The nine polyhedral Zr-0 distances range from 2.10 to 2.56 A at ambient pressure, which represents both an increase in the average and the minimum Zr-0 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/;Pa 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 initi0 calculations.

The phase transformations and pressure-volume dependence of Hf02 have been investigated at room temperature by angle-dispersive powder x-ray diffraction under high pressure to 50 GPa in a diamond anvil cell. The phase transformation from the monoclinic I (baddeleyite) to orthorhombic phase 11 was observed around 10 GPa. This phase is stable up to 26 GPa where it transforms to a new phase 111 with another orthorhombic unit cell. At about 42 GPa, a third phase transition occurs to phase IV oftetragonal symmetry. The pressure dependences of the cell parameters and volume have been determined. The successive volume discontinuities are 2.5%, 2.5%, and 5%, respectively. The bulk moduli of ali the phases have been calculated from Birch's equation of state and are discussed. The high-pressure phases were found to be metastable at normal pressure. No orthorhombic cotunnite-type structure was observed under pressure at room temperature. Although the structural properties of Hf02 and Zr02 are similar at lower pressures, their evolutions are different above 20 GPa.