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.

Nanosized Bi 2 WO 6 was synthesized by a mild hydrothermal crystallization process. This method allowed obtaining plate-like crystallites of very small thickness (down to 3 nm). The effect of particle size on the structure and properties of Bi 2 WO 6 was studied by X-ray diffraction (XRD), transmission electron microscopy, and Raman and infrared spectroscopies. It has been shown that the orthorhombic distortion decreases with decreasing particle size, but the structure of the smallest crystallites is still orthorhombic. Raman studies have also revealed a very strong intensity decrease for those modes that appear mainly for incident and scattered light polarized perpendicular to the layers. This behavior has been attributed to a decrease in the orthorhombic distortion and a plate-like shape of the nanocrystallites.

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The temperature dependence of the X- and Q-band EPR spectra of Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) containing approximately 1% Cu(2+) is reported. All three molecular g-values vary with temperature, and their behavior is interpreted using a model in which the potential surface of the Jahn-Teller distorted Cu(H(2)O)(6)(2+) ion is perturbed by an orthorhombic "strain" induced by interactions with the surrounding lattice. The strain parameters are significantly smaller than those reported previously for the Cu(H(2)O)(6)(2+) ion in similar lattices. The temperature dependence of the two higher g-values suggests that in the present compound the lattice interactions change slightly with temperature. The crystal structure of the Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) host is reported, and the geometry of the Zn(H(2)O)(6)(2+) ion is correlated with lattice strain parameters derived from the EPR spectrum of the guest Cu(2+) complex.

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.

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The crystal structure of the paraelastic phase of KFe(MoO4)2 at 360 K was reinvestigated and high pressure Raman scattering experiments were performed on this material. The studies indicated that this molybdate crystallizes in the structure above 312 K. At room temperature the structure is monoclinic and it transforms under pressure into , and low symmetry phases at 0.25, 1.3 and 1.6 GPa, respectively. The phase transitions observed at 0.25 and 1.6 GPa are irreversible whereas the 1.3 GPa transition is reversible. The lattice dynamics calculations performed for the phase allowed us to obtain an assignment of observed modes and helped us to obtain insight into the mechanism driving the structural changes occurring in this material. The x-ray study of the highest pressure phase, recovered during the decompression experiment, shows that the crystal structure of this phase is monoclinic or triclinic. When this phase is subjected to heat treatment at 673 K, it either transforms into another phase or decomposes.