Abstract
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.