The structure of nanosized clusters of noble-gas atoms undergoes a transition from face-centered cubic (fcc) to hexagonal close packed (hcp) as the clusters’ size approaches 100,000 atoms, according to a study published in Physical Review Letters (DOI: 10.1103/physrevlett.109.245505). The finding resolves a long-standing discrepancy between theoretical predictions and experimental observations. It also deepens understanding of the structures of simple crystals and solid phases—prerequisites to understanding more complex ones. Computations show that weakly interacting atoms such as neon and argon prefer, energetically speaking, to adopt the hcp structure as clusters grow into crystals. But laboratory studies have long shown that the clusters solidify in the fcc structure. New work shows that the discrepancy may have resulted from experimental techniques that limit the clusters’ structure options. To overcome that limitation, Nina V. Krainyukova of the National Academy of Sciences of Ukraine and colleagues injected argon into helium in a way that enables argon atoms to coalesce into small clusters that eventually fuse into larger ones. X-ray diffraction analysis shows that the larger clusters order into the predicted hcp structure through coalescence of fcc-structured small clusters.