Hidden degrees of freedom in aperiodic materials

Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid- solid phase transition of an incommensurate host- guest system: the channel inclusion compound of nonadecane/ urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other- an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal nature's capacity to explore the increased number of phases allowed in aperiodic crystals.