  The superconducting series, Fe(Te,Se), has a complex structural and magnetic
phase diagram that is dependent on composition and occupancy of a secondary
interstitial Fe site. In this letter, we show that superconductivity in
Fe1+xTe0.7Se0.3 can be enhanced by topotactic deintercalation of the
interstitial iron, demonstrating the competing roles of the two iron sites.
Neutron diffraction reveals a flattening of the Fe(Te,Se)4 tetrahedron on Fe
removal of iron and an increase in negative thermal expansion within the ab
plane that correlates with increased lattice strain. Inelastic neutron
scattering shows that a gapped excitation at 6 meV, evolves into gapless
paramagnetic scattering with increasing iron; similar to the fluctuations
observed for non-superconducting Fe1+xTe itself.
