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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 47; no. 3; pp. 879 - 897
• Main Authors: Tavassoli, A, Grytsiv, A, Rogl, G, Romaka, V. V, Michor, H, Reissner, M, Bauer, E, Zehetbauer, M, Rogl, P
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 15.01.2018
• Subjects: Antimony; Cobalt; Crystal structure; Curie temperature; Electrical resistivity; Figure of merit; Iron; Iron 57; Magnetic measurement; Nomograms; Residual resistivity; Seebeck effect; Site preference (crystals); Substitutes; Ternary systems; Thermal conductivity; Tin compounds; Titanium compounds
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c7dt03787b

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 TiFe 1.33 Sb. This compound also exhibits Fe/Ti substitution according to Ti 1+ x Fe 1.33− 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 TiFe 1.375 Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti 1+ x Fe 1.33− x Sb, and of the Fe/Co substitution within the isopleth TiFe 1.33 Sb-TiCoSb. The calculated residual resistivity of Ti 1+ x Fe 1.33− x Sb, as well as of the isopleths TiFe 1.33 Sb-TiCoSb, TiFe 0.665 Co 0.5 Sb-TiCoSb 0.75 Sn 0.25 and TiFe 0.33 Co 0.75 Sb-TiCoSb 0.75 Sn 0.25 , 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 TiFe 1.33 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 Ti 1+ x Fe 1.33− x Sb, TiFe 1.33 Sb-TiCoSb, TiFe 0.665 Co 0.5 Sb-TiCoSb 0.75 Sn 0.25 and TiFe 0.29 Co 0.78 Sb-TiCoSb 0.75 Sn 0.25 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 Ti 1+ x Fe 1.33− 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 TiFe 1.33 Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo 0.5 Fe 0.665 Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe 0.665 Co 0.5 Sb-TiCoSb 0.75 Sn 0.25 and TiFe 0.33 Co 0.75 Sb-TiCoSb 0.75 Sn 0.25 . However, ZT -values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments). Phase equilibria for Heusler Phase Ti 1+ x Fe 1.33− x Sb at 800 °C and calculated isosurfaces ( = 0.42) of the electron localization function in TiFe 1.25 Sb.