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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The crystal structures of the cotunnite-type phases (space group, Pnam, 2 = 4) of pure zirconia and hafnia prepared under high-temperature, high-pressure conditions in a multianvil device were refined by the-of-flight neutron powder Mraction. The structures of both compounds are very similar and the nine polyhedral metal-oxygen distances range from 2.133(1) to 2.546(1) di in Zr02 and from 2.121(1) to 2.535(2) di in Hf02. The Raman spectra of both phases resemble one another strongly and are consistent with the cotunnite-type structure. These results confirm that ZrOz and €NOz undergo transitions to the same phase at high pressure.

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Microporous AlPO 4 -54•xH 2 O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H 2 O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H 2 O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO 4 -54•xH 2 O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IV Al, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

The main models proposed in the literature to describe pressure-induced amorphization (PIA) are briefly reviewed, with special emphasis on the kinetic aspects of PIA. The high potential of Raman spectroscopy for experimental studies on PIA is highlighted and illustrated by the results obtained for zirconium tungstate (ZrW 2 O 8 ) and boric acid (H 3 BO 3 ). Compared with x-ray diffraction using a conventional x-ray tube, Raman spectroscopy allowed a much faster identification of the transitions in both compounds. Further, the Raman results for ZrW 2 O 8 have given support to an amorphization mechanism based on the freezing of low-energy vibrational modes associated with rigid (or quasi-rigid) ZrO 6 and WO 4 polyhedral units. For H 3 BO 3 , we obtained strong evidence of a PIA associated with hindered decomposition of H 3 BO 3 , probably to a mixture of HBO 2 and H 2 O. The short acquisition time for the Raman measurements of samples under high pressure enabled us to follow the kinetics of the transition on H 3 BO 3 , which couldhardly be done by conventional x-ray diffraction. Raman spectroscopy proved to be a very convenient technique to study the mechanisms behind PIA, owing not only to the fast acquisition of information about phase transition kinetics, but also to the structural information that can be obtained.

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.

We have performed all-electron ab initio calculations for TiB 2 in the athermal limit using the CRYSTAL95 code. The lattice parameters of the AlB 2 -type structure were optimized as a function of pressure. The fitting of a Murnaghan equation of state resulted in values of B 0 = 292±1 GPa and B 0 = 3.34±0.03 for the bulk modulus and its first derivative at zero pressure. The values for the linear bulk modulus along the a-axis and the c-axis are B a0 = 1031 ± 3 GPa (B a0 = 10.6 ± 0.2) and B c0 = 675 ± 3 GPa (B c0 = 8.8 ± 0.2), respectively. All five independent elastic constants were calculated, and the analysis of the elastic behaviour of titanium diboride indicates that this compound is more isotropic than one would suppose from its crystal structure. The discussion on the nature of the chemical bonds and the electronic charge transfer in titanium diboride gives some insight into its mechanical properties, such as its high hardness, despite an apparent layered structure. In this sense, the analysis of the charge-density distribution shows a nonnegligible interaction between graphite-like boron planes along the c-axis, which increases with pressure, and suggests a three-dimensional picture for the TiB 2 structure, instead of the traditional planar description.