Polymorphism and superconductivity in the V-Nb-Mo-Al-Ga high-entropy alloys

Abstract

High-entropy alloys (HEAs) are at the focus of current research for their diverse properties, including superconductivity and structural polymorphism. However, the polymorphic transition has been observed only in nonsuperconducting HEAs and mostly under high pressure.Here we report the discovery of superconductivity and temperature-driven polymorphism in the (V0.5Nb0.5)3−xMoxAl0.5Ga0.5 (0.2 ≤ x ≤ 1.4) HEAs. It is found that the as-cast HEA is of a single body-centered cubic (bcc) structure for x = 0.2 and a mixture of the bcc and A15 structures for higher x values. Upon annealing, the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibits bulk superconductivity. For x = 0.2, whereas the bcc polymorph is not superconducting down to 1.8 K, the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and and estimated zero-temperature upper critical field Bc2(0) of 20.1 T, both of which are the highest among HEA superconductors. With increasing Mo content x, both Tc and Bc2(0) of the A15-type HEAs decrease, yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range. The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength. Furthermore, the valence electron count dependence of Tc, which is different from both the binary A15 and other structurally different HEA superconductors, suggests that Tc may be increased further by reducing the number of valence electrons. Our results not only uncover HEA superconductors of a new structural type, but also provide the first example of polymorphism dependent superconductivity in HEAs. PACS numbers:

Superconductivity in Cubic A15-type V–Nb–Mo–Ir–Pt High-Entropy Alloys

“…This points to a decrease of the a-axis with the increase of V content, in consistent with its smaller atomic radius compared with those of Nb and Mo [22]. In addition to the A15 phase, small impurity peaks are observed in the vicinity of main (102) diffraction and probably comes from the NbAl 2 -type sigma phase [18]. In the A15 structure, there are two crystallographic sites (0, 0, 0) and (0.25, 0, 0.5).…”

“…Despite the presence of strong chemical disorder, some HEAs exhibit collective quantum phenomena such as superconductivity [6,7]. So far, a number of HEA superconductors have been discovered and their crystal structures can be categorized into body-centered cubic (bcc)-type [8][9][10][11], a-Mn-type [12,13], CsCl-type [14], hcp-type [15][16][17], A15-type [18], and s-type [19]. In particular, the A15-type V 1.4 Nb 1.4 Mo 0.2 Al 0.5 Ga 0.5 HEA has a T c of 10.2 K and a disorder-enhanced upper critical field of 20.1 T [18], both of which are the highest among HEA superconductors.…”

Superconductivity in hexagonal Nb-Mo-Ru-Rh-Pd high-entropy alloys

“…These HEAs exhibits a weak metallic behavior below 300 K, and the residual resistivity ratio ρ 300K /ρ 7K is only around 1.1. This is reminiscent of that observed in many other HEAs [25][26][27] and ascribed to the presence of strong atomic-scale disorder. With increasing Nb content x, the ρ magnitude decreases strongly, which is probably due to the increase in either carrier concentration or mobility.…”

Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research

“…The values of T c strongly depend on the cubic lattice parameter a and VEC: there is a linear increase of T c as a decreases and VEC increases [36]. Table 4) [37]. The crystal structure of cubic A15 is presented in Figure 1b or Figure 1c.…”