In Are films ferroelectric? | Gold nanocrystals | Photosynthetic system | Dissecting the atom | SAXS and the water channel | Digging in the dirt | Folding Protein Sensors | X-ray movies
In
Are films ferroelectric? |
Gold nanocrystals |
Photosynthetic system |
Dissecting the atom |
SAXS and the water channel |
Digging in the dirt |
Folding Protein Sensors |
X-ray movies
The smallest of structural changes can lead to significant effects on the
properties of technologically important materials. Take perovskites, for
instance. The perovskite structure is an archetypal crystal structure found
in a diverse range of minerals, including materials with useful catalytic
and electronic properties. But, subtle distortions from the archetypal
structure can turn an active catalyst into an inactive one or a useful
superconducting material into an insulator.
Now, Clare Grey, Peter Chupas and their colleagues at the State University
of New York at Stony Brook, Brookhaven National Laboratory, Michigan State
University, and Argonne National Laboratory have used high-energy x-rays
from the 1-ID beamline at the Advanced Photon Source to study the
structure-dependent properties of the perovskite-related mineral a-aluminum
trifluoride (a-AlF3). Their findings show how new x-ray diffraction methods
can allow subtle changes in structure to be detected and modeled more
accurately than before.
a-AlF3 is commonly used in the chemical industry as a catalyst for
fluorocarbon manufacture and as an additive for improving the electrolysis
of aluminum ore in aluminum production. The material has a distorted
perovskite structure related to the compound rhenium oxide at room
temperature. However, heat it above 468 Celsius and the material changes,
adopting the cubic rhenium oxide structure.
According to the team, using conventional approaches, such as powder
diffraction and neutron diffraction, to analyze this phase change does not
provide enough detailed information as to how the change occurs. Instead,
such techniques yield useful long-range information that is averaged out and
so cannot reveal the dynamics of the process. By necessity, it is the
movement of ions or atoms that underlies the structural change. Now, the
team has obtained data from the 1-ID beamline that is detailed enough to
reveal the dynamics of such structure-changing processes.
Previous researchers explained the structural changes in perovskites and
other minerals in terms of the rotation of rigid octahedral sub-units in the
crystal structure. Indeed, scientists use the rigid unit model with great
success to describe the phase changes in silica and various natural
perovskites. In the case of aluminum trifluoride, these octahedra comprise a
central aluminum atom surrounded by six fluorine atoms, one at each vertex
of the octahedron. By combining two distinct techniques - so-called Rietveld
refinement and a pair distribution function (PDF) - Grey, Chupas, and their
colleagues, could observe the shifting octahedra within the material's
structure at temperatures just below the phase change, at the temperature at
which it occurs, and just above it.
The team has also run molecular dynamics simulations in parallel with their
refinement of the beamline data. The simulations gave them a way to
"animate" the phase transition as one form of aluminum trifluoride is
converted into the other with increasing temperature. The simulations then
allowed the researchers to refine their model still further by comparing the
simulation with their experimental results.
The study shows that the high-temperature structure of aluminum trifluoride
is highly dynamic and is essentially composed of a superposition of tilted
"AlF3" octahedra that rotate between the different tilted structures,
leading to an apparent cubic, undistorted structure.
The researchers now hope to extend their approach to other materials such as
other perovskites and catalysts such as cerium dioxide, negative thermal
expansion materials, such as zirconium molybdate, and perovskite minerals of
geological relevance. One such mineral, magnesium silicate, comprises the
bulk of the earth's mantle and is the subject of investigations to explain
anomalous seismic behaviour observed at the boundary between the mantle and
the earth's core. The researchers anticipate that the combined experimental
approach, where both local atomic structure and long-range structure are
probed, will provide valuable insight into how local structural distortions
couple with physical properties or reactivity in this and other related
mineral structures.
See: Peter J. Chupas,1,5 Santanu Chaudhuri,1 Jonathan C. Hanson,2 Xiangyun
Qiu,3 Peter L. Lee,4 Sarvjit D. Shastri,4 Simon J. L. Billinge,3 and Clare
P. Grey,1 "Probing Local and Long-Range Structure Simultaneously: An In Situ
Study of the High-Temperature Phase Transition of a-AlF3," J. Am. Chem. Soc.
126, 4256-2257 (2004).
Additional reference: Santanu Chaudhuri, Peter J. Chupas, Mark Wilson, Paul
Madden, Clare P. Grey, J. Phys. Chem. B. 108, 3437-3445 (2004).
In Issue 76
Are films ferroelectric?
Discipline for gold nanocrystals
X-rays shed light on machinery of photosynthesis
Back to the January elemental discoveries index page