Revealing how interactions lead to ordering in para-terphenyl.

How does the ordering in a crystal arise from the interactions present? Crystal structure determination shows what the crystal structure is, but in solving directly for atomic coordinates leaves questions as to why (or how) aspects of the structure arise. The answers to such questions are crucial in the study of what drives structural phase transitions or in crystal structure prediction. In this work, modeling of the neutron diffuse scattering from deuterated para-terphenyl, C18D14, shows directly how the observed short-range order arises out of intramolecular and intermolecular interactions. This approach provides real-space descriptions of cooperative molecular motions and correlations, and explains the two-dimensional critical behavior observed in other experiments. In doing so, it is found that intramolecular and intermolecular interactions, and the molecules themselves, can be thought of as acting as nanoscale mechanical linkages.