Melting-point depression by insoluble impurities: a finite size effect.

Melting-point depression by soluble impurities is an entropy-driven phenomenon. Studying partially oxidized free sodium nanoparticles, we found an additional mechanism, which is caused by insoluble impurities. Oxidization of sodium clusters with 135-192 atoms by a single oxygen molecule causes a melting-point depression of 17+/-6 K; additional oxygen amplifies the effect. This is in contrast to the behavior of bulk sodium, where the melting point does not change upon partial oxidization. First-principles density functional theory calculations reveal the interaction between the pure and the oxidized part of the cluster to be responsible for the effect. There is more structural freedom in a liquid cluster to optimize the interface between the two parts. This stabilizes the liquid phase and causes the observed, cluster-specific melting-point depression.