Determination of atomic positions and polar direction in half-Heusler material Sb1-xSnxTi1-y-zHfyZrzCo using electron channeling

Variation of electron-induced x-ray emission with the crystal direction of an incoming electron beam (channeling) is used to study the crystal site preference of different elements in a promising thermoelectric half-Heusler material. This material has Sb, Co and Ti as the main constituting elements, with lower contents of Hf, Zr and Sn. It is confirmed experimentally, in accordance with previous assumptions, that Sb and Sn occupy one of the octahedral sites, 4a (0, 0, 0), Ti, Zr and Hf the other octahedral site, 4b (½, ½, ½), and Co atoms occupy the tetrahedral site, 4c (¼, ¼, ¼) in the cubic spacegroup F4¯3m with lattice constant a = 0.598 ± 0.001 nm. Furthermore, a strong asymmetry along the polar directions is observed, which is as it should be in a half-Heusler material, when the Co atoms have a strong preference for one of the two sets of tetrahedral sites. Such asymmetries can not be observed by kinematical diffraction according to Friedel’s law, thus demonstrating that the observation of element characteristic signals under channeling conditions is a unique technique for determining the sense of polar directions and distinguishing crystal symmetry alternatives in small crystal grains and nanocrystals.