The half Heusler system Ti_1+xFe_1.33−xSb–TiCoSb with Sb/Sn substitution: phase relations, crystal structures 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 stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFeSb. This compound also exhibits Fe/Ti substitution according to TiFeSb (-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 TiFeSb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region TiFeSb, and of the Fe/Co substitution within the isopleth TiFeSb-TiCoSb. The calculated residual resistivity of TiFeSb, as well as of the isopleths TiFeSb-TiCoSb, TiFeCoSb-TiCoSbSn and TiFeCoSb-TiCoSbSn, are in a good correlation with the experimental data. From magnetic measurements and Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFeSb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11μ per Fe. Thermoelectric (TE) properties were obtained for the four isopleths TiFeSb, TiFeSb-TiCoSb, TiFeCoSb-TiCoSbSn and TiFeCoSb-TiCoSbSn 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 TiFeSb 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 TiFeSb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCoFeSb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFeCoSb-TiCoSbSn and TiFeCoSb-TiCoSbSn. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

Structural, Chemical, Electrical, and Thermal Properties of n-Type NbFeSb

“…The authors also remark that the distribution of the 1.33 Fe on the 4d sites is not perfectly random. 37 Our observation of a single line spectrum in NbFeSb indicates that the majority of Fe occupies a 4d site without occupation of surrounding vacancy sites.…”

Decisive role of interstitial defects in half-Heusler semiconductors: An ab initio study

“…It is noteworthy that most of the previous ab initio investigations considered defect-free stoichiometric composition for half-Heusler alloys. However, the half-Heusler alloys have been found to be prone to chemical off-stoichiometry [52,53]. The intrinsic point defects resulting in off-stoichiometric compositions have been previously reported for several half-Heusler alloys such as TiNiSn [54][55][56][57], HfNiSn [58,59], and ZrNiSn [33,60,61].…”

How Severe Plastic Deformation Changes the Mechanical Properties of Thermoelectric Skutterudites and Half Heusler Alloys

“…Half-Heusler Alloys P-and n-type half-Heusler alloys were prepared using an optimized arc melting technique followed by a treatment in a high frequency furnace, long time BM and an advanced hotpressing technique (for details see Refs. (Gürth et al, 2016;Tavassoli et al, 2017;Tavassoli et al, 2018)). The HP samples were then HPT processed (HP + HPT).…”

“…The quality of a TE material is judged by the figure of merit, ZT (S 2 T)/(ρλ) where S is the Seebeck coefficient, T is the temperature, ρ is the electrical resistivity, λ λ e + λ ph is the thermal conductivity, consisting of the electronic, λ e , and the phonon part, λ ph . Besides silicides (Sadia et al, 2016), Bi-tellurides (Ma et al, 2008;Poudel et al, 2008;Yu et al, 2009;Ashida et al, 2012), PbTe (Cohen et al, 2015;Guttmann et al, 2015;Komisarchik et al, 2016) or Zintl phases [ (Kauzlarich, 2019) and references therein], just to point out some important TE materials, skutterudites [review articles (Snyder and Toberer, 2008), (Nolas et al, 1999;Uher, 2001;Rowe, 2006;Rull-Bravo et al, 2015;Schierning et al, 2015;Hasan et al, 2020) and references therein] and half-Heusler alloys [ (Sakurada and Shutoh, 2005;Gelbstein et al, 2011;Poon et al, 2011;Xie et al, 2012;Fu et al, 2013;Schwall and Balke, 2013;Appel and Gelbstein, 2014;Fu et al, 2015;Gürth et al, 2016;Li et al, 2016;Tavassoli et al, 2017;Tavassoli et al, 2018;Zhu et al, 2019) and review articles (Casper et al, 2012;Poon, 2018) and references therein] are the most promising candidates, exhibiting not only a high ZT value, but they are easy to fabricate and the starting materials can generally be chosen from those available and cheap. In addition, skutterudites as well as half-Heusler alloys are suitable for heat sources at moderate temperatures (300 -900 K) such as in automobile exhaust or industrial waste heat.…”