Superconductivity in high and medium entropy alloys based on MoReRu

Hexagonal close-packed (HCP) high entropy or medium entropy alloys (HEAs or MEAs) based on alterations of the ternary superconductor Mo_.33Re_.33Ru_.33 (valence electron count VEC = 7/atom) are tested for their potential superconductivity. Quaternary Mo_.25Re_.25Ru_.25Rh_.25 has VEC 7.5 and is superconducting at 2.5 K, implying that its electron count is too high. To decrease the VEC, Ti is added to the mixture, yielding (Mo_.2375Re_.2375Ru_.2375Rh_.2375)Ti_.05 (VEC = 7.325) and (Mo_.225Re_.225Ru_.225Rh_.225)Ti_.1 (VEC = 7.15), which are single phase superconducting HCP HEAs, with superconducting transition temperatures (T_c’s) of 3.6 K and 4.7 K, respectively. Materials with a significant excess of Ru over Mo_.25Re_.25Ru_.25Rh_.25 were also studied; Mo_.1Re_.1Ru_.55Rh_.1Ti_.15 (VEC = 7.20) is HCP, with a T_c of 2.1 K. Decreasing the ruthenium content to Mo_.105Re_.105Ru_.527Rh_.105Ti_.158 (VEC = 7.158) and Mo_.118Re_.118Ru_.470Rh_.118Ti_.176 (VEC = 7.06) yields superconductivity, but with a secondary CsCl-type phase present; T_c’s are 2.2 K and 2.5 K, respectively. Upon further decrease in ruthenium content, to Mo_.167Re_.167Ru_.25Rh_.167Ti_.25 (VEC = 6.674), although the T_c is relatively high at 5.5 K, three phases are present – a dominant sigma phase plus minor CsCl-type and HCP phases. These results suggest that an optimal valence electron count for this HCP based HEA/MEA alloy system is near 7.0, and that the superconducting T_c will not likely get above 10 K.