Melting, freezing, sublimation, and phase coexistence in sodium chloride nanocrystals

Calorimetry measurements, performed by multicollision induced dissociation, have been used to probe the melting of a number of (NaCl)nNa+ clusters with n=22-37. The clusters anneal at 225-325 K and melt at 750-850 K. (NaCl)22Na+ and (NaCl)37Na+, which can adopt geometries that are perfect fragments of the bulk lattice melt at around 850 K. The other clusters, which (except for n=31) must have defects, melt at temperatures which are up to 100 K lower than the perfect nanocrystals. The internal energy distributions become bimodal near the melting temperature. This is the signature of slow dynamic phase coexistence where clusters spontaneously jump back and forth between the solid and liquid states with an average period that is longer than required for thermal equilibration. The jump frequency must be between 10(4) and 10(7) s(-1) for the bimodal distribution to be observable in our experiments. The (NaCl)nNa+ clusters can dissociate by an unusual thermally activated process where melting and freezing raise the internal energy to generate hot solid clusters that can sublime before they cool to the ambient temperature.