The structural and physical properties of the new titanium- and niobium-rich type-A high-entropy alloy (HEA) superconductor Nb${}_{0.34}$Ti${}_{0.33}$Zr${}_{0.14}$Ta${}_{0.11}$Hf${}_{0.08}$ (in at.%) were studied by X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. In addition, electronic structure calculations were performed using two complementary methods: the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) and the Projector Augmented Wave (PAW) within Density Functional Theory (DFT). The results obtained indicate that the alloy exhibits type II superconductivity with a critical temperature close to 7.5 K, an intermediate electron–phonon coupling, and an upper critical field of 12.2(1) T. This finding indicates that Nb${}_{0.34}$Ti${}_{0.33}$Zr${}_{0.14}$Ta${}_{0.11}$Hf${}_{0.08}$ has one of the highest upper critical fields among all known HEA superconductors.

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.

The structural and superconducting properties of a new niobium- and titanium-rich high-entropy alloy (NbTi) $$_{\text {0.67}}$$0.67 (MoHfV) $$_{\text {0.33}}$$0.33 were determined. The alloy was synthesized by arc melting and its physical properties were characterized by means of X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity and specific heat measurements. Experimental data revealed that the (NbTi) $$_{\text {0.67}}$$0.67 (MoHfV) $$_{\text {0.33}}$$0.33 crystalizes in body-centered cubic structure and exhibits type-II superconductivity below about 5 K. In addition, electronic structure calculations were performed using the Density Functional Theory (DFT) method. Their results suggest that in case of HEAs, the influence of the local atomic configuration on their electronic structure in the energy region close to E $$_{\mathrm {\text {F}}}$$F is rather small or even negligible.

We systematically investigate three body-centred cubic (bcc) high-entropy alloys (HEAs) with an identical valence electron count (VEC = 4.375): ( NbHf ) 0.75 ( TiZr ) 0.25 , ( NbZr ) 0.75 ( TiHf ) 0.25 , and ( NbTi ) 0.75 ( ZrHf ) 0.25 . Despite their isoelectronic nature, it was found that these alloys exhibit markedly different superconducting properties. Critical temperatures T c range from 7.14(5) K to 8.17(5) K, while upper critical fields μ 0 H c 2 ( 0 ) exceed 12 T, reaching 14.0(9) T for the Ti-rich alloy ( NbTi ) 0.75 ( ZrHf ) 0.25 . These variations highlight the limited predictive power of the Matthias rule in HEA design. Furthermore, the exceptional properties observed in the Ti-rich composition suggest that Nb and Ti are critical elements for the development of new HEAs for next-generation superconducting magnets.