Long-range order of defects revealed by Mossbauer powder diffracometry

The improvement in our M\"{o}ssbauer powder diffractometer enabled measurement of multiple diffraction patterns at Doppler velocities across all nuclear resonances in a partially-ordered $^{57}$Fe$_{3}$Al. The GCSAXI program was developed to convert detector images to diffraction patterns and then extract the intensities of fundamental and superlattice diffractions. MDS, a M\"{o}ssbauer diffraction simulation program, was developed based on kinematical diffraction theory to calculate M\"{o}ssbauer diffraction intensities from given structures. The MEF program was developed to compare experimental data to calculations and search for best long-range order and short-range order parameters by using a nonlinear fitting alogrithm. The diffraction intensities from the $^{57}$Fe$_{3}$Al sample were analyzed by these three programs to provide data on the long-range order of Fe atoms having different numbers of Al neighbors. It was shown that Fe atoms having three Al atoms as first-nearest neighbors (1nn) are partially simple-cubic ordered, similar to that of Fe atoms with four Al 1nn. The ordered state of Fe atoms with three Al 1nn can be mostly accounted for by homogeneous antisite disorder and antiphase domain boundaries. Further improvement of the performance of our M\"{o}ssbauer powder diffractometeer is under way by exploring the possibility of adopting CdZnTe position sensitive detector.