|
These aperiodic, rule-bending crystals are the focus of an article
co-authored by Hollingsworth that appeared in
the Jan. 4 (2008) issue of the journal Science.
Building on results from Hollingsworth's collaborator, French
researcher Bertrand Toudic, Hollingsworth, Toudic and their co-authors
looked at how these aperiodic crystals behave differently from
"normal" periodic crystals. These differences could have implications
not only for research but also for technology that relies on crystals,
from computer displays to hard drives, Hollingsworth said.
For the research featured in the Science article, Hollingsworth and
colleagues looked at crystals that form a host-guest structure. In
this case, urea molecules formed tunnels around nonadecane molecules,
making a honeycomb-like structure that takes the form of a
double-helix - the shape of DNA. In periodic host-guest crystals,
Hollingsworth said the host molecules forming the tunnels and the
guest molecules inside form a regularly repeating structure. But not
so with the rule-breaking aperiodic crystals.
"Sometimes the host and guest fit nicely, sometimes they don't,"
Hollingsworth said. "This can have a huge effect on all sorts of
properties. During crystal growth, for example, periodic and aperiodic
host-guest crystals can behave very differently."
In aperiodic crystals, in which the host and guest structures don't
match, the guest molecules protrude from the ends of the crystals,
making the surface rough. This means it's easier to attach new
molecules to the end of the crystal. Such crystals, including the ones
featured in the Science article, are shaped like long needles.
But it really gets weird when the crystals undergo transitions from
one phase to another.
"Bertrand and I have been talking about this for years, trying to find
out what's going on in this system," Hollingsworth said. "The idea of
studying these systems is to better understand how phase transitions
work in aperiodic materials."
To find out what's going on in the phase transitions, the researchers
observed the crystals at different temperatures above the phase
transition, when the guest molecules are moving rapidly inside their
tunnel-like hosts, and also at extremely cold temperatures as
molecules are becoming frozen in place. To probe the crystals, the
researchers scattered neutrons from them and measured different types
of reflections. One class of reflections, called satellite reflections,
measures the interaction between the guest and host molecules.
The researchers were surprised by what happened when the crystal was
cooled to about -190 degrees Fahrenheit. The satellite reflections
showed a change in the interaction between the host and guest
structures but no noticeable changes in either the host or guest
structures themselves.
"Previously, we thought these materials had homogenous phase
transitions and that the normal rules concerning symmetry breaking
applied to them," Hollingsworth said. "I don't think anyone would have
predicted what happens in this phase transition."
Because these aperiodic materials don't play by the same rules,
Hollingsworth said the impact on research is that scientists need to
figure out what rules these aperiodic crystals are playing by in phase
transitions. In addition to affecting research, these different rules
also could have impacts on technology, he said. Crystals like the ones
featured in the Science article are ferroelastic. That means that the
molecules within the crystals reorient when the crystals are squeezed.
The researchers can do this with a small anvil and observe the
rotations of large domains in the crystals by viewing the crystal
under a microscope. Closely related ferroelectric materials are
important to technology because the domains within these materials can
be reoriented with electric fields to allow or prohibit polarized
light to pass through. This makes them useful in electronic displays.
"The question is whether these phases that we have observed will have
unusual properties that are useful," he said.
As research on aperiodic crystals continues, Hollingsworth said that
researchers expect this same unusual phase transition behavior in
materials other than the urea-nonadecane crystals used in this study.
|
