Phases and Valence Electron Counts Of Superconducting High Entropy Alloys: A Focus on HCP Structures

Abstract

Many known superconductors suffer structural instabilities; single phase high entropy alloys (HEAs) with high superconducting critical temperatures (Tc), however, are novel materials that can be used under extremely high pressures, inside jet engines, at the Earth’s core, or on surfaces of larger planets, and may replace conventional superconductors altogether. Hexagonal close-packed (HCP) HEAs have recently been discussed for their structural stability, and homogeneity in phase is critical in HEAs to ensure uniformity of property throughout the alloy. In this thesis, I report the superconducting properties of unprecedented single phase HCP HEAs synthesized by arc melting and analyzed by powder X-ray diffraction (pXRD) and Physical Property Measurement System (PPMS). Phase, chemical composition, mole fraction, and valence electron count (VEC) are explored to understand Tc in the larger context of preferential phase selection of HEAs. I show that the Tix(MoReRuRh)1-x HEA series remains single phase HCP until x = 0.1 unless the mole fraction is broken and ruthenium content is increased. This implies ruthenium plays a critical role in retaining the single phase HCP structure in the Ti-Mo-Ru-Re-Rh combination. The analogous Zrx(MoReRuRh)1-x series yields mixed HCP and Mn(a) phases at all values of x. The Mn(a) phase persists despite changes to the mole fraction, suggesting a direct correlation between zirconium and formation of the Mn(a) phase in the Zr-Mo-Re-Ru-Rh combination. Finally, I propose a negative linear correlation between VEC and Tc for HEAs that are single phase HCP or HCP-containing mixed phases as a tool to fine-tune the desired Tc of a superconducting HCP HEA.