LAUSR

Abstract 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 (Tc’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 Tc 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; Tc’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 Tc 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 Tc will not likely get above 10 K.