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

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