Structural and physical properties of the high-entropy alloy ${\left(\mathrm{NbTa}\right)}_{0.67}{\left(\mathrm{MoHfW}\right)}_{0.33}$ were studied by x-ray powder diffraction, energy dispersive x-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. The experimental results were supported by theoretical calculations using two complementary approaches for electronic structure calculations: the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) and projector augmented wave (PAW) within the density functional theory. It was found that the alloy forms with a cubic, body-centered structure (space group $Im\overline{3}m$, W-type structure) with a lattice parameter $a=3.287\left(1\right)$ Å and a random (but microscopically homogeneous) distribution of the constituent elements. At high temperature the alloy exhibits simple metallic behavior, while at low temperature it becomes a type-II superconductor with the critical temperature $T_{\mathrm{c}}\approx 4.3$ K and the upper critical field ${\mu }_0{H}_{\mathrm{c}2}\approx 1.45$ T. The electron-phonon coupling constant calculated from the PAW data, ${\lambda }_{\mathrm{el}-\mathrm{ph}}^{\mathrm{th}-\mathrm{PAW}}=0.63$, is in perfect agreement with the experimental results.

Th-containing superconducting high entropy system with the nominal composition (NbTa) $$_{0.67}$$0.67 (MoWTh) $$_{0.33}$$0.33 was synthesized. Its structural and physical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, specific heat, resistivity and magnetic measurements. Two main phases of alloy were observed: major bcc structure and minor fcc. The experimental results were supported by numerical simulation by the DFT Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA).

A comparison of the results of studies on the influence of structural defects on the critical parameters of superconductivity has been made for the high-entropy alloy (NbTa)${}_{0.67}$(MoHfW)${}_{0.33}$ and for the conventional superconducting magnet material Nb-47wt%Ti. Positron annihilation lifetime spectroscopy (PALS), electrical resistivity, magnetization and magnetic susceptibility measurements were used. In addition, X-ray powder diffraction studies were performed on the high-entropy alloy (NbTa)${}_{0.67}$(MoHfW)${}_{0.33}$. Due to the rapid cooling of the materials after melting in the arc furnace, they contain a higher concentration of structural defects compared to the heat-treated materials. Magnetic property measurements showed that both the critical temperatures $T_{\mathrm{c}}$ and the upper critical fields ${\mu }_0{H}_{\mathrm{c}2}$ of bulk superconductivity-related materials are improved in the presence of structural defects.