Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb$_3$Sn Superconductor Wires

TL;DR

This study uses EXAFS to analyze dopant site occupancy in Nb$_3$Sn superconductors, revealing that antisite disorder and dopant distribution significantly influence high critical current density performance.

Contribution

It provides new insights into the lattice site occupancy of Ti and Ta dopants in Nb$_3$Sn, linking antisite disorder to enhanced high field performance.

Findings

Ta occupies both Nb and Sn sites, Ti primarily occupies Nb sites

Best Ti-doped samples are sub-stoichiometric in Sn

Antisite disorder correlates with high H$_{c2}$ and J$_c$

Abstract

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 Nb$_3$Sn 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 EXAFS to determine the lattice site location of dopants in modern high-performance Nb$_3$Sn 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.