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.
Toudic
Bertrand
B
Universitéde Rennes 1, Institut de Physique de Rennes (IPR), 35042 Rennes Cedex, France. bertrand.toudic@univ-rennes1.fr
Garcia
Pilar
P
Odin
Christophe
C
Rabiller
Philippe
P
Ecolivet
Claude
C
Collet
Eric
E
Bourges
Philippe
P
McIntyre
Garry J
GJ
Hollingsworth
Mark D
MD
Breczewski
Tomasz
T