Physical properties of {Ti,Zr,Hf}NiSn compounds.

Physical properties, 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 single-phase compounds with slightly nonstoichiometric compositions: TiNiSn, ZrNiSn, and HfNiSn. From X-ray single crystal and TEM analyses, TiNiSn, ∼ 0.13(1), is isotypic with the UPtSn-type (space group 4/, ternary ordered version of the ZrAl-type), also adopted by the homologous compounds with Zr and Hf. For all three polycrystalline compounds (relative densities >95%) the electrical resistivity of the samples is metallic-like with dominant scattering from static defects mainly conditioned by off-stoichiometry. Analyses of the specific heat curves and / reveal Sommerfeld coefficients of = 14.3(3) mJ mol K, = 10(1) mJ mol K, = 9.1(5) mJ mol K and low-temperature Debye-temperatures: LTD = 373(7)K, 357(14)K and 318(10)K. Einstein temperatures were in the range of 130-155 K. Rather low Seebeck coefficients (<15 μV K), power factors (pf < 0.07 mW mK) and an estimated thermal conductivity of < 148 mW cm K yield thermoelectric figures of merit ZT < 0.007 at ∼800 K. Whereas for polycrystalline ZrNiSn elastic properties were determined by resonant ultrasound spectroscopy (RUS): = 171 GPa, = 0.31, = 65.5 GPa, and = 147 GPa, the accelerated mechanical property mapping (XPM) mode was used to map the hardness and elastic moduli of TNiSn. Above 180 K, ZrNiSn reveals a quasi-linear expansion with CTE = 15.4 × 10 K. The calculated density of states is similar for all three compounds and confirms a metallic type of conductivity. The isosurface of elf shows a spherical shape for Ti/Zr/Hf atoms and indicates their ionic character, while the [NiSn] sublattice reflects localizations around the Ni and Sn atoms with a large somewhat diffuse charge density between the closest Ni atoms.