The structural compression mechanism and compressibility of gallium oxyhydroxide, α -GaOOH, are investigated by in situ synchrotron radiation x-ray diffraction at pressures up to 31.0 GPa by using the diamond anvil cell technique. The α -GaOOH sustains its orthorhombic structure when the pressure is lower than 23.8 GPa. The compression is anisotropic under hydrostatic conditions, with the a-axis being most compressible. The compression proceeds mainly by shrinkage of the void channels formed by the coordination GaO3（OH）3 octahedra of the crystal structure. Anomaly is found in the compression behavior to occur at 14.6 GPa, which is concomitant with the equatorial distortion of the GaO3（OH）3 octahedra. A kink occurs at 14.6 GPa in the plot of finite strain f versus normalized stress F, indicating the change in the bulk compression behavior. The fittings of a second order Birch-Murnaghan equation of state to the P-V data in different pressure ranges result in the bulk moduli B0=199（1） GPa for P 〈 14.6 GPa and B0=167（2） GPa for P 〉 14.6 GPa. As the pressure is increased to about 25.8 GPa, a first-order phase transformation takes place, which is evidenced by the abrupt decrease in the unit cell volume and b and c lattice parameters.
The structural transition of BaF2 nanocrystals is studied by in situ high pressure synchrotron radiation X-ray diffraction measurements up to about 21.2 GPa at ambient temperature. Two phase transformations were observed at 5.8 and 14.4 GPa, and the two high pressure phases are identified as orthorhombic （Pnma） phase and hexagonal （P63/mmc） phase by Rietveld refinement. Upon decompression, the 0c-PbC12-type metastable phase is retained when the pressure is released. Two phase transformations of the BaF2 nanocrystals are higher than that in bulk BaF2. It is proposed that the size effects are found to influence the BaF2 nanocrystals high-pressure behaviors and the surface energy plays a significant role in the structural stability.
Uniform InN nanowires were studied under pressures up to 35.5 GPa by using in situ synchrotron radiation x-ray diffraction technique at room temperature. An anomalous phase transition behavior has been discovered. Contrary to the results in the literature, which indicated that In N undergoes a fully reversible phase transition from the wurtzite structure to the rocksalt type structure, the In N nanowires in this study unusually showed a partially irreversible phase transition. The released sample contained the metastable rocksalt phase as well as the starting wurtzite one. The experimental findings of this study also reveal the potentiality of high pressure techniques to synthesize In N nanomaterials with the metastable rocksalt type structure, in addition to the generally obtained zincblende type one.
