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.
[1] Using novel experimental methods, we measured the acquisition of isothermal remanent magnetization, direct field demagnetization, and alternating field demagnetization of multi-domain (MD) and single domain (SD) magnetite under hydrostatic pressures to 6 GPa. We find that the saturation remanence of MD magnetite increases 2.8 times over initial, non-compressed values by 6 GPa, while its remanent coercivity remains relatively constant. For SD magnetite, remanent coercivity and saturation remanence vary little from 0 to 1 GPa, increase markedly from 1 to 3 GPa, then plateau above 3 GPa. These new findings suggest that by 3 GPa, SD magnetite either undergoes a magnetic phase transition, or that it reaches an optimal magnetic state where magnetostriction and/or magnetocrystalline energy constants attain some threshold state without reorganization of the pre-existing magnetic lattices. Similar behavior is not observed in MD magnetite, likely due to domain wall effects.
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