Interplay of structure, magnetism, and superconductivity in Se substituted iron telluride with excess Fe.

We investigated the evolution of the temperature-composition phase diagram of Fe1+y Te upon Se substitution. In particular, the effect of Se substitution on the two-step, coupled magneto-structural transition in Fe1+y Te single crystals is investigated. To this end, the nominal Fe excess was kept at y = 0.12. For low Se concentrations, the two magneto-structural transitions displayed a tendency to merge. In spite of the high Fe-content, superconductivity emerges for Se concentrations x >= 0.1. We present a temperature-composition phase diagram to demonstrate the interplay of structure, magnetism and superconductivity in these ternary Fe-chalcogenides.