Physical properties of {Ti,Zr,Hf} 2 Ni 2 Sn compounds.

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 single-phase compounds with slightly nonstoichiometric compositions: Ti 2.13 Ni 2 Sn 0.87 , Zr 2.025 Ni 2 Sn 0.975 , and Hf 2.055 Ni 2 Sn 0.945 . From X-ray single crystal and TEM analyses, Ti 2+ x Ni 2 Sn 1- x , x ∼ 0.13(1), is isotypic with the U 2 Pt 2 Sn-type (space group P 4 2 / mnm , ternary ordered version of the Zr 3 Al 2 -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 C p vs. T and C p / T vs. T 2 reveal Sommerfeld coefficients of γ Ti 2 Ni 2 Sn = 14.3(3) mJ mol -1 K -2 , γ Zr 2 Ni 2 Sn = 10(1) mJ mol -1 K -2 , γ Hf 2 Ni 2 Sn = 9.1(5) mJ mol -1 K -2 and low-temperature Debye-temperatures: θ 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 -1 ), power factors (pf < 0.07 mW mK -2 ) and an estimated thermal conductivity of λ < 148 mW cm -1 K -1 yield thermoelectric figures of merit ZT < 0.007 at ∼800 K. Whereas for polycrystalline Zr 2 Ni 2 Sn elastic properties were determined by resonant ultrasound spectroscopy (RUS): E = 171 GPa, ν = 0.31, G = 65.5 GPa, and B = 147 GPa, the accelerated mechanical property mapping (XPM) mode was used to map the hardness and elastic moduli of T 2 Ni 2 Sn. Above 180 K, Zr 2 Ni 2 Sn reveals a quasi-linear expansion with CTE = 15.4 × 10 -6 K -1 . 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 [Ni 2 Sn] n - sublattice reflects localizations around the Ni and Sn atoms with a large somewhat diffuse charge density between the closest Ni atoms.