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Local atomic dynamics by the Dynamic PDF method

Wojtek Dmowski 1,  Takeshi Egami 1,2

1. University of Tennessee (UTK), Knoxville, TN, United States
2. Oak Ridge National Laboratory (ORNL), One bethel Valley Road, Oak Ridge, TN 37932, United States

Abstract
Advanced neutron facilities allow use of the pair distribution function to study new aspects of local atomic structure and even dynamics.  In this presentation we will describe application of the dynamic pair distribution function (D-PDF) to the study of localized atomic dynamics in a disordered relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN). Lattice dynamics is usually studied by examining phonon dispersion. However, there are special cases when the phonons are localized, and phonon dispersions are ill defined. Recently, we have introduced a new approach using a Dynamic PDF (D-PDF) that overcomes this limitation. The dynamic structure factor S(Q,w) of a powder sample is measured using a time-of-flight inelastic neutron spectrometer, and then Fourier transformation over Q is performed to obtain G(r,w) or D-PDF. In our study of PMN the D-PDF was determined at various temperatures by
pulsed neutron inelastic scattering using the Pharos spectrometer of the Los Alamos Lujan Center. The D-PDF succeeded in characterizing the onset of local dynamic polarization at the Burns temperature, and its freezing at lower temperatures. We had shown that local dynamic polarization resulted in the formation of polar nano-regions and relaxor behavior. The local polarization was observed via D-PDF as a dynamic off-centering of Pb ions. Also, we measured D-PDF for a polycrystalline Ni to demonstrate the principle of the D-PDF. We observed that D-PDF captured high symmetry, non-dispersive phonons near the Van Hove singularity. Figure 1 shows the D-PDF for a polycrystalline Ni at 300K (W. Dmowski et. al,PRL 100, 137602, 2008)       
This work was supported by the NSF grant DMR06-02876 and by the U.S. DOE under DE-AC05-00OR-22725.

 

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Presentation: Oral at 11th European Powder Diffraction Conference, Microsymposium 5, by Wojtek Dmowski
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-30 22:51
Revised:   2009-06-07 00:48