Single-crystal deuterium NMR study of the symmetry breaking in an incommensurate organic inclusion compound

Abstract

A single-crystal deuterium NMR study of the phase transition of the incommensurate organic inclusion compound of n-nonadecane in urea is presented. In agreement with diffraction measurements, we found that the urea host sublattice is not significantly modulated in the high-symmetry phase. Whereas at room temperature the NMR profiles reflect the fast reorientational disorder of the guest molecules, we report a temperature-dependent inhomogeneous broadening of the resonance lines on approaching ${T}_{c}$ from above. This effect is assigned to a pretransitional clustering of the guest molecules and a heuristic model is developed to describe it. At very low temperature below ${T}_{c},$ NMR measurements give direct information on the mode of ordering of the alkane sublattice inside the urea frame. Remaining static disorder could be attributed to the incommensurate feature of the composite crystal.