Frustrated pretransitional phenomena in aperiodic composites

This paper reports on symmetry breaking in the aperiodic inclusion compound n-octadecane/urea and its isotopomer n-octadecane/urea-d4.The high-symmetry phase is described by a hexagonal rank-4 superspace group. Pretransitional phenomena in this crystallographic superspace reveal competing short-range-ordering phenomena within the high-symmetry phase. Very high-resolution diffraction data show that critical scattering appears at inequivalent points within the four-dimensional Brillouin zone, although the first phase transition at Tc1 near 158 K implies the condensation at only one of those points. The resulting superspace group remains of dimension 4. Two other phase transitions are reported at Tc2 = 152.8(4) K and Tc3 = 109(4) K in n-octadecane/urea-d4. The two low-symmetry phases that arise are described by rank-5 superspace groups.