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The grain-size effect on phase transition induced by pressure in ZnO nanocrystals has been investigated by in situ high-pressure synchrotron radiation X-ray powder diffraction, optical and electrical resistance measurements. The transition pressure of the B4-to-B1 phase transformation for 12 nm ZnO is found to be 15.1 GPa while it is 9.9 GPa for bulk ZnO. Three components: the ratio of the volume collapses, the surface energy difference, and the internal energy difference, governing the change of transition pressure in nanocrystals, are uncovered. The enhancement of transition pressure in ZnO nanocrystals as compared with the corresponding bulk material is mainly caused by the surface energy difference between the phases involved. The high-pressure B1 ZnO phase is not metallic in the pressure range up to 18 GPa at room temperature.

Synchrotron radiation X-ray diffraction studies of γ-Fe2O3 have been performed with emphasis on the pressure-induced phase transformation γ(maghemite) → α(hematite) and the equation of state of the nanophase material. For γ-Fe2O3 the bulk modulus has been found to increase from 203 GPa for the bulk material to 305 GPa for 9 nm size crystals. At the same time the transition pressure decreases from 35 to 27 GPa. The reduced transition pressure is explained in terms of nucleation and growth, the larger volume change upon transition in the nanocrystalline material being the main factor.

The crystal structures of rutile (TiO2) and its highpressure polymorphs have been studied by X-ray powder diffraction in the pressure range up to 60 GPa. At 12GPa, rutile transforms to a phase with the baddeleyite (ZrO2) structure. Upon decompression, this phase transtbrms at 7 GPa to another phase with the ~-PbO2 structure. At ambient conditions, the ~-PbO2-type phase is 2.1 (3)% denser than rutile and the baddeleyite-type phase is 11.3 (9)% denser than rutile. In the pressure range of the rutile-to-baddeleyite transition, the difference in density between the two phases is 9.75 (15)%. The zero-pressure bulk moduli, as determined from the equation of state, are 230 (20), 260 (30) and 290 (20)GPa for rutile, the c~-PbO2-type phase and the baddeleyite-type phase, respectively.

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Synchrotron-radiation X-ray diffraction studies of CaF2 at high pressures have been performed on a powder sample up to 45 GPa and on a single-crystal sample up to 9.4 GPa. The bulk modulus of the low-pressure phase was determined to be B o = 87 (5) GPa. A phase transition was observed at about 9.5 GPa. The transition is accompanied by a volume contraction of 11%. The high-pressure phase is orthorhombic PbC12 type (space group Pbnm). The sample only partially reverts to the low-pressure phase upon release of pressure.