Melting and freezing of spherical bismuth nanoparticles confined in a homogeneous sodium borate glass

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The melting temperature and the crystallization temperature of Bi nanoclusters confined in a sodium borate glass were experimentally determined as functions of the cluster radius. The results indicate that, on cooling, liquid Bi nanodroplets exhibit a strong undercooling effect for a wide range of radii. The difference between the melting temperature and the freezing temperature decreases for decreasing radius and vanishes for Bi nanoparticles with a critical radius $R=1.9\text{ }\text{nm}$. The magnitude of the variation in density across the melting and freezing transitions for Bi nanoparticles with $R=2\text{ }\text{nm}$ is 40% smaller than for bulk Bi. These experimental results support a basic core-shell model for the structure of Bi nanocrystals consisting of a central crystalline volume surrounded by a structurally disordered shell. The volume fraction of the crystalline core decreases for decreasing nanoparticle radius and vanishes for $R=1.9\text{ }\text{nm}$. Thus, on cooling, the liquid nanodroplets with $R<1.9\text{ }\text{nm}$ preserve, across the liquid-to-solid transformation, their homogeneous and disordered structure without crystalline core.