This work presents experimental investigation of the thermoelectric and structural characteristics of new double half Heusler alloys Hf_2-xTi_xFeNiSb₂ (x= 0, 0.15, 0.25 and 0.4). The studied samples were synthesized using different techniques of melt spinning, arc melting and spark plasma sintering. Crystal structure of the studied compounds was examined via x-ray diffraction (XRD). A face centered cubic structure was found to be the dominant crystallization phase. Homogeneity and relatively high density were obtained in the fabricated samples. Thermoelectric properties of the samples were studied over a temperature range from 300 to 800 K. Thermal conductivity was found to be significantly decreased with the Titanium (Ti) doping due to enhanced the phonon scattering. The highest value of the thermoelectric figure of merit was obtained for Hf_1.75Ti_0.25FeNiSb₂ to be 0.28±0.014 at 800 K.

Structural, electronic, and magnetic properties of Mn₂Co_1-xV_xZ (Z = Ga, Al, x = 0, 0.25, 0.5, 0.75, 1) Heusler alloys were theoretically investigated for the case of L2₁ (space group Fm$\overline{3}$m), L2_1b (L2₁ structure with partial disordering between Co and Mn atoms) and XA (space group F$\overline{4}$3m) structures. It was found that the XA structure is more stable at low V concentrations, while the L2₁ structure is energetically favorable at high V concentrations. A transition from L2₁ to XA ordering occurs near x = 0.5, which qualitatively agrees with the experimental results. Comparison of the energies of the L2_1b and XA structures leads to the fact that the phase transition between these structures occurs at x = 0.25, which is in excellent agreement with the experimental data. The lattice parameters linearly change as x grows. For the L2₁ structure, a slight decrease in the lattice constant a was observed, while for the XA structure, an increase in a was found. The experimentally observed nonlinear behavior of the lattice parameters with a change in the V content is most likely a manifestation of the presence of a mixture of phases. Almost complete compensation of the magnetic moment was achieved for the Mn₂Co_1-xV_xZ alloy (Z = Ga, Al) at x = 0.5 for XA ordering. In the case of the L2₁ ordering, it is necessary to consider a partial disorder of atoms in the Mn and Co sublattices in order to achieve compensation of the magnetic moment.

So far, only basic magnetic and transport properties of Mn ₂ CoSb Heusler alloy have been reported in the literature. In this work, we investigated magnetotransport properties of melt-spun ribbons of Mn ₂ CoSb. Calculation of normal Hall coefficients R ₀ revealed a dominant role of electrons in electrical transport of the alloy at temperatures below 200 K. At room temperature, the hole-like conductivity predominates. Dependence of the anomalous Hall coefficient R _S on electrical resistivity ρ _xx , R _S ~ ρ _xx ² , points to the mechanism of the anomalous Hall effect, independent of the concentration of the scatterers. Pronounced hysteretic features of magnetoresistivity Δρ _xx were observed upon application and removal of the magnetic field at temperatures below 100 K. It was suggested that the observed hysteresis of Δρ _xx is related to a magnetic after-effect in the region of randomly frozen spin states.