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On the Half Heusler system Ti1+xFe1.33-xSb - TiCoSb with Sb/Sn substitution: phase relations, crystal structure and thermoelectric properties

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 stoichiometry of TiFeSb (reported previously in the literature) to the Fe-rich composition of TiFe1.33Sb. This compound also exhibits Fe/Ti substitution resulting in a chemical formula Ti1+xFe1.33-xSb -0.17 < x < 0.25 at 800°C). Stability, crystal symmetry and site preference were established by X-ray powder techniques backed by DFT calculations. The ab- initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms of 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, heat capacity, magnetic susceptibility for Ti1+xFe1.33-xSb, as well as for the isopleths TiFe1.33Sb - TiCoSb, TiFe0.665Co0.5Sb - TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb - TiCoSb0.75Sn0.25 are in a good correlation with experimental data. From magnetic measurements and 57Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K is obtained for TiFe1.33Sb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11 µB/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) in the temperature range from 300 K to 823 K allowing to calculate the dimensionless figure of merit (ZT). Composition dependences for the TE properties for all these series were derived. Although p-type Ti1+xFe1.33-xSb indicates a semi-conducting like 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 in this series 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 obtained for 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 optimization of the preparation route (ball-mill conditions and heat treatments).

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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., 2017, Accepted Manuscript
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    On the Half Heusler system Ti1+xFe1.33-xSb - TiCoSb with Sb/Sn substitution: phase relations, crystal structure and thermoelectric properties

    A. Tavassoli, A. Grytsiv, G. Rogl, V. Romaka, H. Michor, M. Reissner , E. Bauer, M. Zehetbauer and P. F. Rogl, Dalton Trans., 2017, Accepted Manuscript , DOI: 10.1039/C7DT03787B

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