Structural, electronic, and magnetic properties of Mn2Co1-xVxZ (Z = Ga, Al, x = 0, 0.25, 0.5, 0.75, 1) Heusler alloys were theoretically investigated for the case of L21 (space group Fmm), L21b (L21 structure with partial disordering between Co and Mn atoms) and XA (space group F3m) 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 L21b 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 Mn2Co1-xVxZ 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.

The characteristics related to electricity and magnetism in Heusler alloys with both full (L21) and inverse (XA) structures X2FeSi (X = Mn, V) have been studied within the framework of the Density Functional Theory. Three different methods, namely LDA, GGA, and SCAN, were used to perform calculations. The aim was to investigate the energy stability of the L21 and XA structures for these compositions. The findings revealed that the XA structure is energetically stable for both structures. The choice of functional is indicated does not have a qualitative effect on the energy stability of the phases. Based on calculations, it was found that meta-GGA (SCAN) more accurately describes the electronic properties of these alloys. In the process of the calculations, it was found that these compounds are semimetals. An analysis was conducted from a local environment perspective to investigate and understand the reasons behind the semi-metallic band gap and the variations in electronic and magnetic properties observed in Heusler compounds. Calculations also showed that the magnetic moment Mn2FeSi for both structures was 1.99 µB/f.u. With regard to V2FeSi, µ = 2.00 µB/f.u. for structure XA and µ = 2.37 µB/f.u. for structure L21. These calculations are consistent with the Slater-Pauling rule for the XA structure.