For 35 years, almost all applications of the Mossbauer effect in materials science have utilized its capabilities as a spectroscopy. Many investigators have identified and quantified spectral components from different crystallographic phases or chemical environments of the Mossbauer atom. Mossbauer scattering can also be coherent, however, enabling its use for a fourth type of diffraction experiment on materials (the other three being X-ray, electron, and neutron diffraction). The unique feature of Mossbauer diffraction is its spectroscopic selectivity. This program studies disorder in metallic alloys by the chemical environment selectivity of Mossbauer diffraction. The chemical sensitivity of Mossbauer spectrometry is used to select an atom in a particular chemical environment, and a diffraction pattern is then measured from atoms having that particular environment. An area detector based on a CCD camera is now available, and funds are requested for a new type of gas-filled area detector developed by a group in Frankfurt. Studies are on alloys of 57Fe3Al and 57FeRh having imperfect chemical order. In these studies, measurements are made of the spatial periodicities of irregular chemical environments (Fe sites other than the Fe sites of the DO3 and B2 ordered structures). With the Frankfurt detector, it should be possible to measure diffraction patterns from different Fe environments in a quasicrystalline Al-Cu-Fe alloy. If detector issues are resolved, some synchrotron experiments on Mossbauer diffraction will be performed during the course of this research. In the last couple of years, advances at third generation synchrotron sources have made it possible to measure nuclear excitations accompanied by phonon excitations. These inelastic spectra can be used to obtain the phonon partial densities of states of Fe atoms in the material. This work on inelastic nuclear resonant scattering will show how vibrations of Fe atoms contribute to the vibrational entropy of different alloy phases. Inelastic nuclear resonant scattering recently showed that the vibrational entropy of Fe3Al depends almost entirely on chemical short-range order (as opposed to long-range order). This proposal describes measurements to be made on how the vibrations of Fe atoms depend on chemical order in Pt3Fe and FeRh, how Fe vibrations depend on point defect concentrations in FeAl, and how Fe vibrations differ for quasicrystalline and crystalline Al-Cu-Fe. %%% After approximately 25,000 publications on Mossbauer spectrometry studies of materials, the Mossbauer effect is well established and well suited for many studies in materials science. Its extension to inelastic nuclear resonant spectrometry should be useful for studies of atom vibrations in small samples such as thin films for which neutron methods are not practical. The complementary nature of the three diffraction methods (X-ray, electron, and neutron) has sustained their widespread use in materials science. There is room for a fourth method of Mossbauer diffractometry.
