[Zurück]

A. Tavassoli, A. Grytsiv, G. Rogl, V.V. Romaka, H. Michor, M. Reissner, E. Bauer, M. Zehetbauer, P. Rogl:
"The half Heusler system Ti1+xFe1.33−xSb-TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties";
Dalton Transactions, 47 (2018), S. 879 - 897.

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).

half-Heusler compounds, phase relations, crystal structure, thermoelectric properties

http://dx.doi.org/10.1039/c7dt03787b