Formation and Superconductivity of Single-Phase High-Entropy Alloys with a Tetragonal Structure

High-entropy alloys (HEAs) are random solid solutions consisting of five or more elements arranged in an ordered crystal lattice. So far, however, nearly all the single-phase HEAs have either a cubic or hexagonal structure. Here, we report the observation of a single tetragonal sigma (σ) phase in two series of Ta₅[Mo_35–xW_5+x]­Re₃₅Ru₂₀ and [Ta_5+yMo_35–y]­W₅Re₃₅Ru₂₀ HEAs up to x = 30 and y = 8, respectively. Meanwhile, these HEAs are discovered to exhibit a fully gapped bulk superconductivity with a maximum T_c of 6.29 K at x = y = 0. T_c decreases monotonically with increasing either x or y, which is mainly due to the weakening of the electron phonon coupling strength. Despite the variation in T_c, the superconducting gap remains essentially unchanged for x ≤ 20 and y ≤ 8, but decreases in magnitude at higher x values. Furthermore, it is found that the stability of single σ-phase HEAs is favored by a high valence electron concentration and correlates well with the electronegativity difference of the constituent elements. Our results not only enlarge the number of possible structural symmetries for HEAs but also help to establish the criteria for single σ-phase formation in these multicomponent systems.