May 2, 2003
Structure of V2O5 ·nH2O Xerogel Solved by the Atomic Pair Distribution Function Technique
V. Petkov1, P.N. Trikalitis1, E.S. Bozin1, S.J.L. Billinge1, M.G. Kanatzidis1, and T. Vogt2
1Michigan State University, East Lansing, MI;
2Brookhaven National Laboratory, Upton, NY;
The structure of most materials can be determined because they can be crystallized and thus studied by a technique called crystallography. But disordered materials lack the periodicity of crystals and show diffraction patterns that are much more diffuse. Scientists from BNL and Michigan State University have successfully applied a technique called atomic pair distribution function at NSLS beamline X7A to determine the structure of the V2O5·nH2O xerogel, a disordered material that could be used in chemical sensors, fast switching devices, and reversible lithium ion batteries.
The V2O5·nH2O xerogel, an organic polymer that can swell in
suitable solvents to yield particles possessing a network of polymer
chains, has fascinated researchers for many decades and inspired an
intensive search for potential applications, such as chemical
sensors, fast switching devices, and reversible lithium ion
batteries. Despite decades of experimentation with this xerogel, its
atomic structure has remained somewhat of a mystery because it does
not form crystals but exists only as ribbon-like particles about 10
nanometers wide and one micrometer long, as shown in figure 1.
The limited diffraction pattern of V2O5·nH2O makes it impossible to determine its three-dimensional structure using traditional crystallographic techniques. But the pattern contains a small number of features that indicate the presence of intermediate-range order and a pronounced diffused component. These are characteristics of “nanocrystalline” materials that have well-ordered local structures limited to the nanometer length scale.
Scientists have put forward two competing structural models for
V2O5·nH2O. Jacques Livage of the University of Paris VI proposed
that the xerogel, on the atomic scale, is a stack of corrugated
single layers of VO5 units, with the layers closely related to
those occurring in crystalline V2O5. On the other hand, Y. Oka of
Kyoto University proposed that the xerogel is made of V2O5 bilayers
according to the crystalline structure of either NaxV2O5 or KxV2O5
(where x is an integer). But neither model can fully explain
experimental observations made using the x-ray diffraction
technique nor describe the atomic structure in terms of a unit cell
and atomic coordinates.
We have determined the three-dimensional structure of V2O5·nH2O using the atomic pair distribution function (PDF) technique, which has emerged recently as a powerful and unique tool for the structural characterization of crystalline materials with significant disorder. The strength of the technique is that it takes into account all components of the diffraction pattern and thus reflects the longer-range structural order and the local deviations from it. The technique is gaining importance because of the availability of high energy, high flux synchrotron sources, such as the NSLS, that make accurate and fast data collection possible.
We have shown that the structure of V2O5·nH2O can be well described as an assembly of almost perfect bilayers of single V2O5 layers made of square pyramidal VO5 units with water molecules residing between them, as shown in figure 2. The stacking sequence is imperfect because of the extensive turbostratic disorder in this material, meaning that the layers are stacked in one direction, but rotated every one way in the other two, similar to a deck of cards that has not been straightened after a game.
This structure explains almost all known spectroscopic, physical, and chemical properties of the material, and reveals the atomic ordering in the individual ribbon-like particles, as shown in figure 3.
The most important outcome of the study is that it yields the three-dimensional structure of nanocrystalline V2O5·nH2O in terms of a relatively simple model with only few meaningful parameters, such as its unit cell and symmetry. This work also shows that, even with a very low degree of structural coherence, at synchrotrons such as NSLS, using the right techniques, it is possible to determine nanoscale structures at the atomic level.
Although the PDF technique is not an ab initio structure determination technique, it can successfully distinguish between different structural possibilities, giving both local and long-range structural information.
BEAMLINE
X7A
FUNDING
National Science Foundation
U.S. Department of Energy
PUBLICATION
“Structure of V2O5·nH2O Xerogel Solved by the Atomic Pair
Distribution Function Technique," V. Petkov, E. S. Bozin, S. J. L.
Billinge, T. Vogt, P. N. Trikalitis and M.G. Kanatzidis, J. Am.
Chem. Soc. 2002, 124 (34): 10157-10162
FOR MORE INFORMATION
Mercouri G. Kanatzidis
Department of Chemistry
Michigan State University
East Lansing, MI
Email: kanatzid@cem.msu.edu
Web: http://www.cem.msu.edu/~kanatzid/
Simon J. L. Billinge
Department of Physics and Astronomy
Michigan State University
East Lansing, MI
Email: billinge@pa.msu.edu
Web: http://www.totalscattering.org/