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V.V. Romaka, G. Rogl, V Bursikova, J. Bursik, H. Michor, A. Grytsiv, E. Bauer, G. Giester, P. Rogl:
"Physical properties of {Ti,Zr,Hf}₂Ni₂Sn compounds";
Dalton Transactions, 51 (2022), S. 361 - 374.

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_2.13Ni₂Sn_0.87, Zr_2.025Ni₂Sn_0.975, and Hf_2.055Ni₂Sn_0.945. From X-ray single crystal and TEM analyses, Ti_2+xNi₂Sn_1−x, x ∼ 0.13(1), is isotypic with the U₂Pt₂Sn-type (space group P42/mnm, ternary ordered version of the Zr₃Al₂-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 Cp/T vs. T² reveal Sommerfeld coefficients of γ_Ti2Ni2Sn = 14.3(3) mJ mol⁻¹ K⁻², γ_Zr2Ni2Sn = 10(1) mJ mol−1 K−2, γ_Hf2Ni2Sn = 9.1(5) mJ mol⁻¹ K⁻² and low-temperature Debye-temperatures: θ^LT_D = 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 μ V 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 Zr2Ni2Sn 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 T2Ni2Sn. Above 180 K, Zr2Ni2Sn 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 [Ni2Sn]ⁿ⁻ sublattice reflects localizations around the Ni and Sn atoms with a large somewhat diffuse charge density between the closest Ni atoms.

http://dx.doi.org/10.1039/d1dt03198h