The half Heusler system Ti1+xFe1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties. Issue 3 (19th December 2017)

• Record Type: Journal Article
• Authors: Tavassoli, A.; Grytsiv, A.; Rogl, G.; Romaka, V. V.; Michor, H.; Reissner, M.; Bauer, E.; Zehetbauer, M.; Rogl, P.
• Is Part Of: Dalton transactions. Volume 47:Issue 3(2017)
• Page Start: 879
• Page End: 897
• Publication Date: 2017-12-19
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c7dt03787b
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Phase equilibria for Heusler Phase Ti1+ x Fe1.33− x Sb at 800 °C and calculated isosurfaces ( ϒ = 0.42) of the electron localization function in TiFe1.25 Sb. 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 TiFe1.33 Sb. This compound also exhibits Fe/Ti substitution according to Ti1+ x Fe1.33− x Sb (−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 TiFe1.375 Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+ x Fe1.33− x Sb, and of the Fe/Co substitution within the isopleth TiFe1.33 Sb–TiCoSb. The calculated residual resistivity of Ti1+ x Fe1.33− x Sb, as well as of the isopleths TiFe1.33 Sb–TiCoSb, TiFe0.665 Co0.5 Sb–TiCoSb0.75 Sn0.25 and TiFe0.33 Co0.75 Sb–TiCoSb0.75 Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57 Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33 Sb, with a paramagnetic Curie–Weiss temperature of −8 K and an effective moment of 1.11 μ B per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+ x Fe1.33− x Sb, TiFe1.33 Sb–TiCoSb, TiFe0.665 Co0.5 Sb–TiCoSb0.75 Sn0.25 and TiFe0.29 Co0.78 Sb–TiCoSb0.75 Sn0.25 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+ x Fe1.33− x Sb 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 TiFe1.33 Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5 Fe0.665 Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665 Co0.5 Sb–TiCoSb0.75 Sn0.25 and TiFe0.33 Co0.75 Sb–TiCoSb0.75 Sn0.25 . However, ZT -values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).