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Issue 3, 2018
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The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties

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Abstract

Investigations of phase relations in the ternary system Ti–Fe–Sb show that the single-phase region of the Heusler phase is significantly shifted from stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFe1.33Sb. This compound also exhibits Fe/Ti substitution according to Ti1+xFe1.33−xSb (−0.17 ≤ x ≤ 0.25 at 800 °C). Its stability, crystal symmetry and site preference were established by using X-ray powder techniques and were backed by DFT calculations. The ab initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+xFe1.33−xSb, and of the Fe/Co substitution within the isopleth TiFe1.33Sb–TiCoSb. The calculated residual resistivity of Ti1+xFe1.33−xSb, as well as of the isopleths TiFe1.33Sb–TiCoSb, TiFe0.665Co0.5Sb–TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb–TiCoSb0.75Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33Sb, with a paramagnetic Curie–Weiss temperature of −8 K and an effective moment of 1.11μB per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+xFe1.33−xSb, TiFe1.33Sb–TiCoSb, TiFe0.665Co0.5Sb–TiCoSb0.75Sn0.25 and TiFe0.29Co0.78Sb–TiCoSb0.75Sn0.25 by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (λ) at temperatures from 300 K to 823 K allowing the calculation of the dimensionless figure of merit (ZT). Although p-type Ti1+xFe1.33−xSb indicates a semi-conducting behavior for the Fe rich composition (x = −0.133), the conductivity changes to a metallic type with increasing Ti content. The highest ZT = 0.3 at 800 K was found for the composition TiFe1.33Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5Fe0.665Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665Co0.5Sb–TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb–TiCoSb0.75Sn0.25. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

Graphical abstract: The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties

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Publication details

The article was received on 09 Oct 2017, accepted on 30 Nov 2017 and first published on 30 Nov 2017


Article type: Paper
DOI: 10.1039/C7DT03787B
Citation: Dalton Trans., 2018,47, 879-897
  • Open access: Creative Commons BY license
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    The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties

    A. Tavassoli, A. Grytsiv, G. Rogl, V. V. Romaka, H. Michor, M. Reissner, E. Bauer, M. Zehetbauer and P. Rogl, Dalton Trans., 2018, 47, 879
    DOI: 10.1039/C7DT03787B

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