Evolution of Tetragonal Phase in the FeSe Wire Fabricated by a Novel Chemical-Transformation Powder-in-Tube Process

We fabricated superconducting FeSe wires by the chemical-transformation powder-in-tube (PIT) process. The obvious correlation between annealing temperature and phase transformation was observed. Annealing above 500 °C produced wire-core transformation from hexagonal to tetragonal phase. Furthermore the hexagonal phase completely transformed into the tetragonal phase by annealing at 1000 °C. With increasing annealing temperature, the superconducting property was dramatically improved, associated with the evolution of the tetragonal phase.