Abstract
This letter reports on the structural instabilities of an aperiodic composite crystal under pressure. The (P, T) phase diagram up to 0.55 GPa of nonadecane-urea is reported showing various symmetry breakings in crystallographic superspaces, towards three different orthorhombic phases. These structural phase transitions are characterized by a change in the intermodulation and are described by increasing the rank of the crystallographic superspaces.