Morphological alteration upon phase transition and effects of oxygen impurities of chromium nanoparticles

The morphological alteration from δ- to α-Cr nanoparticles and the phase transition temperature were directly observed by in situ experiment using a transmission electron microscope. The icositetrahedral nanoparticle of δ-Cr transformed into a rhombic dodecahedral nanoparticle of α-Cr upon heating at 550°C. It was demonstrated that the most stable shape of the α-Cr nanoparticle is the rhombic dodecahedron. Upon heating the α-Cr nanoparticle at 650°C, the (1 1 3) layer of Cr2O3 grew topotactically in the direction of [100] of α-Cr nanoparticle. We concluded that the α-Cr nanoparticle includes about 20-30 at% oxygen, as estimated from the volume of the Cr2O3 surface layer.