Phonons from neutron powder diffraction

The spherically averaged structure function S(\q\)obtained from pulsed neut ron powder diffraction contains both elastic and inelastic scattering via a n integral over energy. The Fourier transformation of S(\q\) to real space, as is done in the pair density function (PDF) analysis, regularizes the da ta, i.e., it accentuates the diffuse scattering. We present a technique whi ch enables the extraction of off-center (\q\not equal 0) phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure [dynamics from powder diffraction] has bee n successfully implemented as demonstrated here for two systems, a simple m etal fee Ni and an ionic crystal CaF2. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and addit ionally provides off-center phonon information from neutron powder diffract ion, [S0163-1829(99)05733-1].