UNUSUAL STRUCTURE, PHASE-TRANSITION, AND DYNAMICS OF SOLID CUBANE

The cubic structure of the cubane molecule, C8H8, gives rise to many u nusual properties in the solid stale. X-ray powder diffraction measure ments of solid cubane reveal a large thermal expansion. followed by a strong first-order phase transition at 394 K from an orientationally o rdered phase (<R(3)over bar>) to a noncubic orientationally disordered phase (<R(3)over bar> or R3m). The resulting volume expansion is 5.4% . Model calculations predict the correct low- and high-temperature pha ses. The orientational potential in the disordered phase resembles a r ounded square well over a large angular range. Cubane thus represents an ideal example of a system with large-amplitude collective motions.