Mossbauer diffractometry: principles, practice, and an application to a study of chemical order in 57Fe3Al

Abstract

For the first time, Mossbauer powder diffractometry went beyond the proof-of-principal stage and was used to study unknown periodicities of defect-related chemical environments of Fe atoms in a partially-ordered 57Fe3Al polycrystalline sample. Mossbauer powder diffractometry is based on two phenomena, the Mossbauer effect and the Bragg diffraction. The Mossbauer effect is sensitive to short-range order whereas diffractometry is sensitive to long-range order. Together, they enable Mossbauer powder diffractometry to provide information on long-range periodicities of target atoms having specific short-range order. Both experimental and theoretical efforts are necessary for this novel technique to become practial. In this research, hardware and software for Mossbauer powder diffractometry were improved. A kinematical diffraction theory for Mossbauer powder diffractometry incorporating effects of interference between electronic and nuclear resonant scattering was developed. The applicability of the theory was verified by computer calculations that accounted for dynamical diffraction effects. A thorough analysis of polarization effects, including a polycrystalline average of polarization factors, was done systematically using spherical harmonic expansions. Multiple diffraction patterns were measured at Doppler velocities across all nuclear resonances of 57Fe3Al. On the basis of the theory developed, the superlattice diffractions were analyzed to provide data on the long-range order of Fe atoms having different numbers of Al neighbors. Comparing experimental data to calculations showed that Fe atoms having three Al atoms as first-nearest neighbors (1nn) have partial simple cubic long-range order, similar to that of Fe atoms with four Al 1nn. The simple cubic periodicity of Fe atoms with three Al 1nn was significantly lower than expected for homogeneous antisite disorder, however. Monte-Carlo simulations and transmission electron microscopy suggest that a significant fraction of aperiodic Fe atoms with three Al 1nn are near antiphase domain boundaries.