Solid cubane: A brief review Review Article

It is relatively easy to predict the structural properties of a solid made of spherical molecules. However this is not the case for a solid composed of cubic molecules such as solid cubane. The cubic structure of the cubane molecule gives rise to many unusual solid state properties. Here we present a brief review of our X-ray and neutron scattering studies along with our model calculations of the structure and lattice dynamics of solid cubane. The low-temperature phase (below the transition temperature of 394 K) is rhombohedral with α = 72.69 ° and a = 5.20 Å. Surprisingly, the crystal structure of cubane in the orientationally-disordered phase is not cubic, but instead remains rhombohedral with α = 103.3 °, far from the fcc value of 60 °. The jump in lattice constant at this transition is 0.05 Å, which corresponds to a 5.4% volume expansion, among the largest ever observed. Neutron inelastic scattering measurements indicate that the librons lie higher in energy than do the phonons, but that they soften rapidly with increasing temperature. Remarkably, model calculations predict the correct low and hightemperature crystal structures, and reveal cubane to be a text-book example of a system with anisotropic large-amplitude collective motions in the disordered phase.