ALCHEMI for coexistent Heusler and half-Heusler phases in TiNi1.5Sn

Atomic location by channeling-enhanced microanalysis (ALCHEMI) is an effective technique to clarify the atomic configurations in multicomponent compounds. Recent development of the theory on the characteristic X-ray emission has made ALCHEMI a more reliable and expansive technique. In this revised ALCHEMI, the characteristic X-ray intensities are measured at various electron-incidence directions, and are compared with the X-ray intensities calculated from first principles. In the present work, this technique was applied to coexistent Heusler and half-Heusler phases in a thermoelectric material, TiNi_1.5Sn. The X-ray intensities calculated for both phases were significantly dependent on the fitting parameters of the atomic occupancy on each site and the sample thickness. Using the Levenberg–Marquardt least-squares method, the residual errors between the experimental and the calculated X-ray intensities were minimized to determine the fitting parameters quantitatively. In the Heusler phase, the ordered structure was observed because the atoms of each element almost occupy each site. On the other hand, in the half-Heusler phase, the “Heusler-like” structure was observed because the occupation probabilities of Ni atoms on the Ni-site and on the vacancy-site were measured as 0.49±0.01 and 0.49±0.01, respectively, whereas 1 and 0 is expected for the normal half-Huesler structure.