Irregular variations in the melting point of size-selected atomic clusters

Small particles have a lower melting point than bulk material1. The physical cause lies in the fact that small particles have a higher proportion of surface atoms than larger particles—surface atoms have fewer nearest neighbours and are thus more weakly bound and less constrained in their thermal motion2,3 than atoms in the body of a material. The reduction in the melting point has been studied extensively for small particles or clusters on supporting surfaces. One typically observes a linear reduction of the melting point as a function of the inverse cluster radius2,4,5. Recently, the melting point of a very small cluster, containing exactly 139 atoms, has been measured in a vacuum using a technique in which the cluster acts as its own nanometre-scale calorimeter6,7. Here we use the same technique to study ionized sodium clusters containing 70 to 200 atoms. The melting points of these clusters are on average 33% (120 K) lower than the bulk material; furthermore, we observe surprisingly large variations in the melting point (of ±30 K) with changing cluster size, rather than any gradual trend. These variations cannot yet be fully explained theoretically.