Structural studies of cyclohexane in confined geometry by neutron diffraction

Neutron diffraction measurements have been made for cyclohexane, C6D12, in the confined geometry of a porous silica with a characteristic pore size of 90 angstrom. The results confirm the large depression of the phase transition temperatures from the liquid to plastic crystal and plastic crystal to brittle crystal phase. The results are compared with some previous studies of phase transitions in bulk cyclohexane. It is found that the structural changes follow a systematic variation similar to that of the bulk phase, only at a lower temperature. However, the Bragg peaks of the solid phase are much broader in the confined geometry, and this behavior is not fully explained by diffraction broadening. It seems that there is increased disorder resulting from the restricted volume of the crystallites and a hysteresis effect dependent on the thermal history of the sample. The defective nature of the lattice, which is related to the coupling of the translational displacement to the local orientation of adjacent molecules, appears to be enhanced for the confined cyclohexane in both the plastic and brittle crystal phases.