engleski

Microstructural features of distinct ZnO particles studied via high-pressure induced powder X-ray diffraction

Structural and microstructural features of ZnO particles prepared by different routes were determined by pressure-induced synchrotron powder X-ray diffraction measurements and complemented by the results of optical micrographs showing the morphological diversity of different ZnO polycrystals. In particular, the pressure-induced change in crystallite size and the contributions of internal strain to the mechanical properties of nanocrystalline ZnO were systematically scaled. A transition from wurtzite to rocksalt phase in ZnO with spherical nanoparticles grown hydrothermally from ethanolic solution resulted in a strong ∼21% collapse in unit cell volume at ∼17.68 GPa, whereas compression of spindle-shaped ZnO particles prepared in NaOH with calcination treatment led to the formation of an exclusively cubic phase at 13.77 GPa, reflecting a ∼20% unit cell volume collapse. Interestingly, the fingerprints of the cubic diffraction patterns in spherical ZnO did not reappear upon decompression from ∼29 GPa - the hexagonal structures were preserved beyond the upward transition pressure, whereas in contrast, a very different trend was observed in spindle-shaped ZnO after depressurization, showing a reversible pressure- induced structural transition to single-phase hexagonal symmetry by slowly depressurizing from 15.46 GPa to ambient pressure. These different features point to a rather contradictory revresibility phenomenon that needs to be investigated: why do the phase-pure ZnO samples of the cubic assemblies show different transformation activities at the same pressure ranges? This tricky behavior results from the differences in surface energy between the crystalline phases and the volume collapse ratio. In addition, the particle size significantly affects the pressure range of phase stability and the final ratios of rocksalt and wurtzite phases at ambient conditions in different ZnO samples. The microstrain response of ZnO particles to applied hydrostatic pressure was investigated up to 30 GPa and revealed that the diversity of ZnO particle size and shape determines the reversibility of wurtzite to rock salt transformation by dictating the microstructure-dependent deformation behavior and ultimately leading to different microstrain responses to hydrostatic pressure.

high pressure XRD ; zinc oxide ; phase transitions ; microstructure ; optical micrographs

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