Crystal Growth, Polar Ordering and Domain Switching in Ferroelastoelectric Inclusion Compounds Mark D. Hollingsworth, Chemistry Department, Kansas State University, Manhattan, KS 66506 As part of our attempt to understand the nature of ferroelastic phase transitions in aperiodic materials, we have studied the low temperature phase behavior of series of channel inclusion compounds of urea with Toudic and coworkers at the University of Rennes. Most alkane/urea inclusion compounds are incommensurate; that is, there are no reasonably small integers m and n for which nchost = mcguest, where chost and mcguest are the host and guest repeat distances along the channel axis. For such incommensurate structures, the diffraction patterns are readily treated in a superspace formalism with four dimensions, and Bragg peaks may be separated into four classes: host, guest, common, and satellite. The satellite reflections characterize the modulation of one substructure on the other, and might be expected to change congruently with the host, guest and common reflections as the structure distorts during the ferroelastic phase transition. With nonadecane/urea, cooling to 149K gives rise to a phase transition in which the unit cell doubling in the ab plane occurs only in the satellite peaks, i.e., the ones that characterize the modulation of one substructure by the other. Although the mean structures of host and guest remain the same from channel to channel, the modulation alternates from channel to channel with twofold screw symmetry. This unprecedented behavior shows that unanticipated degrees of freedom must be considered when studying phase transitions in aperiodic materials.