Physical properties of {Ti,Zr,Hf}2Ni2Sn 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: Ti2.13Ni2Sn0.87, Zr2.025Ni2Sn0.975, and Hf2.055Ni2Sn0.945. From X-ray single crystal and TEM analyses, Ti2+xNi2Sn1-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 Cpvs. T and Cp/T vs. T2 reveal Sommerfeld coefficients of γTi2Ni2Sn = 14.3(3) mJ mol-1 K-2, γZr2Ni2Sn = 10(1) mJ mol-1 K-2, γHf2Ni2Sn = 9.1(5) mJ mol-1 K-2 and low-temperature Debye-temperatures: θLTD = 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 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.

The half Heusler system Ti1+xFe1.33-xSb-TiCoSb with Sb/Sn substitution: phase relations, crystal structures and thermoelectric properties.

Investigations of phase relations in the ternary system Ti-Fe-Sb show that the single-phase region of the Heusler phase is significantly shifted from stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFe1.33Sb. This compound also exhibits Fe/Ti substitution according to Ti1+xFe1.33-xSb (-0.17 ≤ x ≤ 0.25 at 800 °C). Its stability, crystal symmetry and site preference were established by using X-ray powder techniques and were backed by DFT calculations. The ab initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+xFe1.33-xSb, and of the Fe/Co substitution within the isopleth TiFe1.33Sb-TiCoSb. The calculated residual resistivity of Ti1+xFe1.33-xSb, as well as of the isopleths TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57Fe Mössbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33Sb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11µB per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+xFe1.33-xSb, TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.29Co0.78Sb-TiCoSb0.75Sn0.25 by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (λ) at temperatures from 300 K to 823 K allowing the calculation of the dimensionless figure of merit (ZT). Although p-type Ti1+xFe1.33-xSb indicates a semi-conducting behavior for the Fe rich composition (x = -0.133), the conductivity changes to a metallic type with increasing Ti content. The highest ZT = 0.3 at 800 K was found for the composition TiFe1.33Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5Fe0.665Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

Novel refractory phase, Ta7Si2(Si(x)B(1-x))2.

X-ray single crystal (XSC) and powder diffraction data (XPD) were used to elucidate the crystal structure of a new refractory silicon boride Ta7Si2(Si(x)B(1-x))2 (x = 0.12). Tetragonal Ta7Si2(Si(x)B(1-x))2 (space group P4/mbm; a = 0.62219(2) nm, c = 0.83283(3) nm) with B atoms randomly sharing the 4g site with Si atoms is isotypic with the boride structure of (Re,Co)7B4. The architecture of the structure of Ta7Si2(Si(x)B(1-x))2 combines layers of three-capped triangular metal prisms (Si,B)[Ta(6+2)(Si,B)] alternating with double layers of two-capped Si[Ta(8+1)Si] Archimedian metal antiprisms. Consequently, the metal framework contains (B/Si) pairs and Si-Si dumbbells. These two types of coordination figures around the nonmetal atoms are typical for the system-inherent structures of Ta2B (or Ta2Si) and Ta3B2. DFT calculations showed strong B(Si)-B(Si) and Si-Si bonding and represent Ta7Si2(Si(x)B(1-x))2 as a covalent-ionic compound. This bonding behavior is reflected in the high hardness value of 1750 HV. The Sommerfeld constant, γ = 7.58 mJ/mol K(2), as derived from the electronic density of states, calculated at the Fermi level, suggests typical metallic behavior.

Dy2Ni7Sn3: a new member of the CaCu5 family of intermetallics.

Single crystals of didysprosium heptanickel tritin were synthesized from the constituent elements by arc-melting. Two of the five Ni atoms are at general sites and all other atoms are at sites with either twofold or m symmetry. The structure contains 'DyNi(5)Sn' and 'DyNi(2)Sn(2)' fragments and represents a new member of the CaCu(5) series of intermetallics.

Zr(3)NiSb(7): a new anti-mony-enriched ZrSb(2) derivative.

Single crystals of trizirconium nickel hepta-anti-monide were synthesized from the constituent elements by arc-melting. The compound crystallizes in a unique structure type and belongs to the family of two-layer structures. All crystallographically unique atoms (3 × Zr, 1 × Ni and 7 × Sb) are located at sites with m symmetry. The structure contains 'Zr(2)Ni(2)Sb(5)' and 'Zr(4)Sb(9)' fragments and might be described as a new ZrSb(2) derivative with a high Sb content.