Chemical Environment Selectivity in Mossbauer Diffraction

We demonstrate a new feature of Mossbauer diffraction that is useful for studies of atomic arrangements in materials --that Mossbauer diffraction can measure the autocorrelation function of ^{57}Fe atoms as a function of their chemical environment. To acquire the experimental data, we built a Debye-Scherrer type powder diffractometer with a ^{57}Co radiation source and a large angle position sensitive detector. By working with polycrystalline materials near the kinematical limit of diffraction, the broadening of nuclear energy levels is not severe, so the spectroscopic capabilities of Mossbauer scattering are preserved. The sample was polycrystalline ^{57}Fe _3Al with the ordered D0 _3 structure. The two sites for the ^{57}Fe atoms, the Wyckoff 4(b) and 8(c) sites, differ in both their chemical environment (0Al versus 4Al 1nn) and in their spatial arrangement (face -centered cubic, fcc, with lattice parameter 2a _0 versus simple cubic, sc, with lattice parameter a_0). Diffraction peaks from the fcc structure were detected when the incident radiation was tuned to the Mossbauer resonance of the Wyckoff 4(b) Fe site, but not for tuning to the 8(c) site, thereby distinguishing the spatial arrangements of these two Fe sites. Thus, the validity of chemical environment selectivity was proven. The phase change of the Mossbauer scattering near resonance affects the interference of diffracted waves from different chemical environments. Interference effects between x-ray Rayleigh scattering and Mossbauer scattering from the ^{57}Fe _3Al sample were observed, as were interference effects involving the different components of the Mossbauer spectra. A simple oscillator model was used successfully to calculate the interference effects seen in the experimental data.