Mössbauer diffractometry on polycrystalline57Fe3Al

Abstract

A Mössbauer powder diffractometer was used to measure diffraction patterns from polycrystalline foils of 57 Fe 3 Al. The intensities of Bragg diffractions were measured as a function of the energy of the incident photon. The bcc fundamental diffractions showed large changes in intensity as the incident energy was tuned through the nuclear resonances. These variations of diffraction intensity with incident energy were calculated with reasonable success using a kinematical theory of diffraction that included effects of coherent interference between x-ray Rayleigh scattering and, more importantly for these samples, Mössbauer scattering from nuclei having different hyperfine magnetic fields.

Site-specific long-range order in57Fe3Al measured by Mössbauer diffractometry

“…The distribution of 57 Fe HMFs was obtained by processing the experimental spectra by the method of Le Cae¨r and Dubois (1979) and is presented in figure 2. In this method a linear dependence of the isomer shift IS on the HMF H was assumed: IS ¼ AH þ B, where A and B were reported previously to be A ¼ À 1.25 Â 10 À3 mm s À1 kG À1 and B ¼ 0.336 mm s À1 (Gao and Fultz 1993).…”

“…After Fe 3 Al is quenched rapidly, subsequent annealing at low temperatures shows a number of peculiarities, such as the formation of the B32 structure, which may be a kinetic transient (Anthony and Fultz 1989, Gao and Fultz 1993, or perhaps driven by magnetic interactions in substoichiometric zones (Becker and Schweika 1996). In alloys with partial order, it is possible to determine sublattice occupancies by the intensities of superlattice diffractions of X-rays or neutrons, although occupancies become difficult to quantify when the state of order is high and the effects on intensity are small.…”

“…In alloys with partial order, it is possible to determine sublattice occupancies by the intensities of superlattice diffractions of X-rays or neutrons, although occupancies become difficult to quantify when the state of order is high and the effects on intensity are small. In many cases including non-equilibrium alloys with small grain sizes, Mo¨ssbauer spectrometry is capable of measuring the defect environments in partially ordered Fe 3 Al, specifically Fe atoms with different numbers of Al first-nearest neighbours (1NNs) (Stearns 1966, Frackowiak 1985, Fultz and Gao 1993. These defect environments may have spatial periodicities of the D0 3 structure, although the problem may not be simple owing to expected statistical non-uniformities of composition or if the defect environments interact.…”

“…The present Mo¨ssbauer diffractometry measurements were planned with the intention of using the single-scattering kinematic theory for data interpretation (O'Conner andBlack 1964, Belyakov 1975). The past decade has seen improvements in both instrumentation and theory for Mo¨ssbauer powder diffractometry in the kinematic limit (Tegze and Faigel 1994, Stephens et al 1994, Stephens 1996, Stephens and Fultz 1997, Fultz and Stephens 1998. Stephens and Fultz (1997) first showed how 57 Fe atoms with specific chemical environments could be selected for diffraction by tuning the energy of the g-ray photons.…”

“…The Mo¨ssbauer powder diffractometry measurements were performed on the same polycrystalline 57 Fe 3 Al sample that was used in proofof-principle experiments where Mo¨ssbauer diffractometry distinguished spatial arrangements of the well-known chemical environments of the ordered D0 3 structure (Stephens and Fultz 1997, Fultz and Stephens 1998. This sample of 57 Fe 3 Al with partial D0 3 order contains Fe atoms with various chemical environments (designated by the number n of 1NN Al atoms), and these different environments have well-separated hyperfine magnetic fields (HMFs).…”