Abstract
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.
Highlights
The search for materials with low thermal conductivity is an important issue within several fields of materials science
The error for the different elements are indicated in the front of the element symbol
Compositional inhomogeneity and crystal site preference have been studied for a half-Heusler material of composition Sb0.8Sn0.2Ti0.5Hf0.25Zr0.25Co, which is a promising p-type thermoelectric material
Summary
The search for materials with low thermal conductivity is an important issue within several fields of materials science. Examples of such materials are thermoelectric materials where atomic disorder may contribute to reduced thermal conductivity, which may result in higher energy efficiency of the materials used in thermoelectric heat pumps and electricity generators. To better understand the lattice (phonon) contribution to thermal conductivity, the crystal site preference for the different elements and the spatial variations in compositions are useful information. Particular attention is given to the practical use of this electron channeling technique ( referred to as ALCHEMI5) to determine the site preferences as well as to addressing the lack of inversion symmetry in half-Heusler materials Y and z values, this material is an excellent p-type thermoelectric material. Particular attention is given to the practical use of this electron channeling technique ( referred to as ALCHEMI5) to determine the site preferences as well as to addressing the lack of inversion symmetry in half-Heusler materials
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