43708-AC10
Crystal Growth, Polar Ordering and Domain Switching in Ferroelastoelectric Inclusion Compounds
A major focus of this grant is to generate a clearer understanding of domain switching in ferroelastic crystals and to develop new materials with improved properties based on that understanding. Much of this research program has focused on urea inclusion compounds (UICs), which have a rich chemistry and physics associated with their domain switching and phase behavior. Most UICs exhibit hexagonal metric symmetry at room temperature. A fraction of these materials are ferroelastic (i.e., distorted in the ab plane) at room temperature. Upon cooling, many of the hexagonal UICs undergo ferroelastic phase transitions (i.e., from paraelastic to ferroelastic), in which one or both of the cell axes perpendicular to the channel (a and or b) undergo distortion and often doubling. In collaboration with our colleagues at the University of Rennes, we initiated a survey of ferroelastic phase transitions in alkane/urea inclusion compounds, which are among the simplest of the one-dimensional host:guest materials. Most of the alkane UICs 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. Understanding the trends and ordering phenomena in these low temperature phase transitions has been crucial to the development of new ferroelastic materials in our laboratory.
Although the locations (Ghkl) of diffraction peaks of typical three-dimensional crystals can be defined with three indices (h, k and l), an additional parameter or parameters must be used to describe the diffraction patterns of aperiodic materials such as incommensurate channel inclusion compounds. Because the periodicities of host and guest substructures differ only along the channel axis (the c axis), this superspace formalism requires four variables to describe the positions of the Bragg diffraction peaks for the system:
Ghklm = h a* + k b* + l chost*+ m cguest*
With channel inclusion compounds, the collinearity of chost and cguest simplifies things greatly. Here, the Bragg peaks may be separated into four distinct classes: peaks from the commensurate (a*, b*) plane are indexed (h k 0 0) and are called common Bragg peaks; host peaks reflect the mean periodicity of the host and are indexed (h k l 0); guest peaks reflect the mean periodicity of the guest and are indexed (h k 0 m); and finally, satellite peaks, which characterize the intermodulation of one substructure on the other, are indexed as (h k l m) with l and m not equal to zero.
We recently described a ferroelastic phase transition in nonadecane/urea in which cooling to 149K gave rise to a diffraction pattern in which the cell axis doubling occurred only in the satellite peaks, i.e., the ones that characterize the intermodulation of one substructure on the other. In the process of cooling through this phase transition, the average positions of host and guest molecules are the same from channel to channel, as shown by the C-centered orthorhombic symmetry displayed by host and guest reflections. However, the intermodulations of one substructure on the other are related by a twofold screw along the a axis.
In contrast with traditional (periodic) materials, many more types of phase transitions may be considered in crystallographic systems requiring four or more dimensions, even when considering only the group/sub-group relations between phases. However, this phase transition is unprecedented in that it utilizes degrees of freedom that are only available in four-dimensional superspace. This result rules out the usual three dimensional interpretation applied to phase transitions in this group of materials, in which a single hexagonal-to-orthorhombic transition was most readily described in terms of a herringbone ordering of the guest molecules coupled to an antiferroelastic shearing of the host.
In collaboration with our partners in Rennes, we have used high resolution X-ray diffraction to study the ferroelastic phase transitions in over a dozen members of this series of alkane UICs. These materials exhibit a diverse array of phenomena as they are cooled through their ferroelastic phase transitions, and one of them clearly shows this same phenomenon of symmetry breaking in superspace, and over a much wider temperature range than the analogous phase in nonadecane/urea. We continue to look for other classes of compounds that exhibit this same phenomenon, in particular, the alkanone/urea systems, which exhibit much more pronounced interchannel ordering of the guests than their hydrocarbon counterparts.
