W0337

Mossbauer Diffractometry Measurements of Site-Specific Long-Range Order in ⁵⁷Fe₃Al. B. Fultz and J. Y. Y. Lin, California Inst. of Technology, W.M. Keck Laboratory, Mail 138-78, Pasadena CA 91125, btf@caltech.edu.

We report, for the first time, measurements on materials structure by Mossbauer diffractometry. The underlying mechanism of coherent scattering uses the nuclear resonance of ⁵⁷Fe, and is fundamentally different from the mechanisms for x-ray, neutron, or electron diffraction. It is highly sensitive to the chemical environment of the scattering nucleus through hyperfine interactions (well-known from conventional Mossbauer spectrometry). In the present work, Mossbauer powder diffractometry was used to study partially-ordered ⁵⁷Fe₃Al. The intensities of fundamental and superlattice Bragg diffractions were measured at 89 Doppler velocities through all nuclear resonances in the sample. The energy spectra of the Bragg diffraction intensities showed detailed structure that could be analyzed to provide data on the long-range order of Fe atoms having different numbers of Al neighbours. Energy spectra of the Bragg diffractions of Mossbauer radiations were calculated with both kinematical theory and dynamical theory. Comparing experimental data to calculations showed that Fe atoms having three and five Al atoms as first-nearest neighbours (1nn) have partial simple cubic long-range order, similar to that of Fe atoms with four Al 1nn. The Fe atoms with two Al 1nn had partial fcc order similar to that of Fe atoms with zero Al 1nn. No evidence was found for B32 order for any of the Fe environments.