Thermoelectric Compounds TZnSb, T = Ti to Ni ?

We present results of a combined experimental and first-principles study on compounds TZnSb (T=Ti to Ni). Physical properties i.e. electrical resistivity (4.2 - 800 K), Seebeck coefficient (300 - 800 K), specific heat (2 - 110 K), Vickers hardness and elastic moduli (RT) have been defined for the afore mentioned polycrystalline single-phase materials. From X-ray single crystal and Rietveld analyses all these phases, of which TiZnSb and Cr_1-xZnSb are novel representatives, were found to crystallize with the ordered MnAlGe-type (space group <i>P</i>4/<i>nmm</i>), whereas NiZnSb is a half Heusler phase. The thermal stability has been investigated by differential thermal analysis.<br/>For the polycrystalline compounds TZnSb (relative densities &gt;90%) the electrical resistivity is gradually changing from typical metallic behaviour (TiZnSb) to semiconductor-like features with resistivities decreasing with rising temperature (T=V,Cr). Specific heat curves Cp vs T and Cp/T vs T² of TiZnSb and CrZnSn conform with a simple metal behaviour revealing Sommerfeld coefficients of 6.4 (1) mJ/molK² and 12.4 (2) mJ/molK² and low-temperature Debye-temperatures of 273(5) K and 245(5) K, respectively.<br/>Rather low Seebeck coefficients (|S_V|&lt;30 µV/K), power factors (pf&lt;0.05mW/mK²) and an estimated thermal conductivity of λ&lt;6 mW/(cmK), yield low thermoelectric figures of merit consistent with the rather metallic behavior of the materials. It should be noted that TiZnSb and VZnSb reveal negative Seebeck values with shallow minima at about 500 K, whereas CrZnSb exhibits a positive Seebeck coefficient which gradually tends towards zero at 800 K.<br/>Indentation hardness and elastic moduli of TZnSb were extracted employing the accelerated mechanical property mapping (XPM) mode and were found to be within 5.5±1&lt;H_V&lt;6±1 GPa and E=100±10 GPa for T=Ti, V, but somewhat lower E=80±10 GPa for CrZnSb. The Poisson ratio in all cases was ν=0.24 (taken from DFT calculations).<br/>A detailed analysis of the structure types TZnSb adopted by the 3d transition elements clearly reveals that electropositive elements T=Ti,V,Cr,Mn,Fe crystallize with the tetragonal MnAlGe-type but with rising electronegativity T=Fe,Co,Ni incline to the cubic half-Heusler structure or to a (2a×2a×2a) superstructure of it. The case of Fe with three modifications constitutes the border range.<br/>A variety of physical properties are derived from first principles such as structural and magnetic structural stabilities, charge transfer and atomic size, elastic properties, electronic structure (density of states, band structure), electronic transport properties (Seebeck coefficient and resistivity) within Boltzmann's transport theory. All the results are obtained for fully relaxed structural parameters. Concerning magnetic ordering the in-plane antiferromagnetic ordering of T-magnetic moments in the tetragonal MnAlGe-type is dominant. The influence of T- vacancies for VZnSb and Cr_1-xZnSb are investigated and the results emphasize the role of such vacancies.