Superconductivity in a new hexagonal high-entropy alloy

High-entropy alloys (HEAs) are a new class of materials with an attractive combination of tunable mechanical and physicochemical properties. They crystallize mainly in cubic structures, however, for practical applications, HEAs with hexagonal-close-packed (hcp) structure are highly desirable in connection with their, in general, high hardness. Herein, we report the synthesis, structure, and detailed superconducting properties of Re0.56Nb0.11Ti0.11Zr0.11Hf0.11—the first hexagonal superconducting HEA composed of five randomly distributed transition metals. A combination of room-temperature precession electron diffraction, precession electron diffraction tomography, and powder x-ray diffraction is utilized to determine the room-temperature crystal structure. Transport, magnetic, and heat capacity measurements show that the material is a type-II superconductor with the bulk superconducting transition at Tc=4.4 K, lower critical field Hc1(0)=2.3 mT, and upper critical field Hc2(0)=3.6 T. Low-temperature specific-heat measurement indicates that Re0.56Nb0.11Ti0.11Zr0.11Hf0.11 is a phonon-mediated superconductor in the weak electron-phonon coupling limit with a normalized specific-heat jump ΔCelγnTc=1.32. Further, hexagonal to cubic structural transition is observed by lowering the valence electron counts and Tc follows crystallinelike behavior.