Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Thermoelectric Half-Heusler compounds TaFeSb and Ta 1-x Ti x FeSb (0 ≤ x ≤ 0.11): Formation and physical properties A. Grytsiv a,b,c,∗ , V.V. Romaka a,d,1 , N. Watson a,2 , G. Rogl a,b,c , H. Michor b , B. Hinterleitner b , S. Puchegger e , E. Bauer b,c , P. Rogl a,c a Institute of Materials Chemistry, University of Vienna, Währingerstrasse 42, A-1090, Wien, Austria b Institute of Solid State Physics, TU Wien, Wiedner Hauptstrasse, 8-10, A-1040, Wien, Austria c Christian Doppler Laboratory for Thermoelectricity, Wien, Austria d Department of Materials Science and Engineering, Lviv Polytechnic National University, 79013, Lviv, Ustiyanovycha Str. 5, Ukraine e Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090, Wien, Austria ARTICLE INFO Keywords: Intermetallics (antimonides) Thermoelectric properties Physical properties Mechanical properties ab-initio calculations ABSTRACT We report on the formation, physical-chemical, as well as elastic and mechanical properties of the novel Half- Heusler (HH) compound TaFeSb that forms during a solid-state reaction from TaSb 2 and TaFe 2 in the tem- perature range between 800 and 850 °C. TaFeSb behaves as a semiconductor, and changes the conductivity type either on temperature or composition. Transport properties of TaFeSb and Ta 1-x Ti x FeSb (0 ≤ x ≤ 0.11) were measured in the temperature range from 4.2 to 823 K, and the effect of titanium on thermoelectric and me- chanical properties of Ta 1-x Ti x FeSb was investigated. The Ta/Ti substitution results in a significant increase of the thermoelectric power factor to exciting values of above 6 mW/m⋅K 2 . In combination with a suppressed phonon thermal conductivity, due to a unique role of Ti, an enhanced figure of merit, ZT 900K =1.0 (for Ta 0.94 Ti 0.06 FeSb) is obtained, close to the highest values reported for Hf-free p-type HH-systems. In addition, experimental results obtained in this study are discussed and analyzed in the context of ab-initio Density Functional Theory (DFT) calculations. 1. Introduction Thermoelectricity, as the simplest way to directly convert waste heat to electricity, has become an important subject particularly in view of requests for a reduction of CO 2 emission and to decrease the de- pendence on un-recovered sources of energy (natural gas, oil and coal). The energy conversion efficiency: = + + + T T T ZT ZT 1 ( ) 1 1 ( ) h c h a a T T c h (1) of thermoelectric materials in general is determined by the di- mensionless figure of merit ZT = S 2 σ/(λ e +λ ph ), where S is the Seebeck coefficient, T the absolute temperature, σ the electrical conductivity, λ e and λ ph the electron and phonon components of the total thermal conductivity λ and (ZT) a is the average ZT value between T c and T h .A high Carnot prefactor, η c = (T h eT c )/T h , needs a large temperature difference between the hot side, T h , and the cold side, T c , of the thermo- electric legs. During the last years considerable efforts have been made to enhance the thermoelectric performance of several material classes, including tellurides, silicides, skutterudites and half Heusler alloys [1–6]. At elevated temperatures (above 600 °C) half Heusler alloys are among the most promising candidates for thermoelectric (TE) devices. Half Heusler (HH) and Heusler (H) phases crystallize in closely related cubic structure types: MgAgAs-type (noncentrosymmetric space group type F m 43 ¯ ; HH) and MnCu 2 Al-type (centrosymmetric space group type Fm m 3 ¯ ; H). The difference in the crystal symmetry affects also the electronic structure of these compounds and electro-neutral compositions (semi- conductors or so-called Zintl compounds [7]) occur with different electronic configurations: 18 and 24 external electrons for HH and H phases, respectively. Half Heusler thermoelectric materials have the advantage of a tuneable electronic structure, which can be modified, e.g. through doping or substitution on the three sublattices and in ad- dition through partial filling of the vacant sites in the https://doi.org/10.1016/j.intermet.2019.04.011 Received 1 November 2018; Received in revised form 3 February 2019; Accepted 7 April 2019 ∗ Corresponding author. Institute of Materials Chemistry, University of Vienna, Währingerstrasse 42, A-1090, Wien, Austria. E-mail address: andriy.grytsiv@univie.ac.at (A. Grytsiv). 1 Present address: Leibniz Institute for Solid State and Materials Research, Helmholtzstr. 20, D-01069, Dresden, Germany. 2 On Leave from University of Warwick, Coventry, CV4 7AL, UK. Intermetallics 111 (2019) 106468 0966-9795/ © 2019 Published by Elsevier Ltd. T