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Condensed Matter Seminars

ics Condensed Matter
Thursday, November 29, 2007
4:00 PM
Physics Building, Room 204
Note special time.
Note special room.
Dr. Thomas Proffen [Host: Despina Louca]
Los Alamos National Laboratory
"Understanding short- and medium range order in materials using total neutron scattering"
 Slideshow (PDF)
Determination of the atomic structure is mainly based on the measurement of Bragg intensities and yields the average structure of the infinite crystalline material. However, this approach ignores any defects or local structural deviations that manifest themselves as diffuse scattering. It also fails in case of disordered materials, badly crystalline such as many nano-materials, or not crystalline at all, such as glasses. In some cases crystalline and amorphous phases coexist making the traditional crystallographic structure refinement difficult or incomplete. The total scattering pattern or the derived atomic pair distribution function (PDF), however, contains structural information over all length scales [1] and can be used to obtain a complete structural picture of complex materials. One of the great advantaged of the PDF is the fact that one can limit the range on atom-atom distance over which the structural model is refined. Focusing on small distances up to a few Angstroms will illuminate the local structure where as refinements over a wide range will yield the medium and long range structure. It is interesting to consider, that instruments such as the high resolution neutron powder diffractometer NPDF located at the Lujan Neutron Scattering Center at Los Alamos National Laboratory allows the measurement of PDFs up to distances in excess of 200Å or 20nm. As a result one can obtain a ‘complete’ structural fingerprint of nanoparticles that are frequently smaller in size as demonstrated in a recent study of gold nanoparticles [2]. [1] Th. Proffen, S.J.L. Billinge, T. Egami and D. Louca, Z. Krist. 218, 132-143 (2003). [2] K.L. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri and A.K. Cheetham, Chem. Phys. Lett. 393, 385-388 (2004).

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