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 TiFe1.33Sb. 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).
Highlights
Thermoelectricity is one of the simplest means of the direct conversion of heat into electricity
0.58816(4) —e a These symbols were used for the samples in all figures in this article. b The composition of the Half Heusler (HH) phase. c After hot-pressing at 950 °C for 1/2 h the pressure was released and the sample was annealed at 800 °C for 4 h. d The low density of this sample is due to the insufficient pressure during hot pressing. e No maximum was observed for the Seebeck coefficient in the investigated temperature range
One can see that the homogeneity region of the HH phase (τ1-Ti1+xFeSb) at 800 °C includes the stoichiometric composition TiFeSb with an extended homogeneity region (−0.2 ≤ x ≤ 0.27)
Summary
The aforementioned conflicting facts for TiFeSb-based thermoelectrics clearly reveal that both sections TiFexCo1−xSb and Ti1−xFexCoSb were ill chosen demanding a re-optimization of thermoelectric properties along proper isopleths These arguments prompted us to focus our investigations in this work on several tasks: (i) to clarify the crystal structure and extension of the single-phase region of the Heusler phase in the ternary system Ti–Fe–Sb; and (ii) to re-investigate the homogeneity regions and to study the effects of Fe/Ti, Fe/Co and Sb/Sn substitutions on the TE properties for the Heusler phase in the system Ti–Fe–Co–Sb–Sn (for detailed location of the samples planned and investigated, see Fig. 1).
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