Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb₃Sn Superconductor Wires

To meet critical current density, J _c , targets for the Future Circular Collider (FCC), the planned replacement for the Large Hadron Collider (LHC), the high field performance of Nb₃Sn must be improved, but champion J _c values have remained static for the last 10 years. Making the A15 phase stoichiometric and enhancing the upper critical field H _c2 by Ti or Ta dopants are the standard strategies for enhancing high field performance but detailed recent studies show that even the best modern wires have broad composition ranges. To assess whether further improvement might be possible, we employed Extended X-ray Absorption Fine Structure (EXAFS) to determine the lattice site location of dopants in modern high-performance Nb₃Sn strands with J _c values amongst the best so far achieved. Although Ti and Ta primarily occupy the Nb sites in the A15 structure, we also find significant Ta occupancy on the Sn site. These findings indicate that the best performing Ti-doped stand is strongly sub-stoichiometric in Sn and that antisite disorder likely explains its high average H _c2 behavior. These new results suggest an important role for dopant and antisite disorder in minimizing superconducting property distributions and maximizing high field J _c properties.