Two-step Transition in the Fe(Se,Te) Phase Diagram

In the studied crystals of FeSe 0.7 Te 0.3 , a structural phase transition occurs in two stages. At higher temperatures, the electronic subsystem undergoes a rearrangement, leading to a significant increase in elastoresistance. 77 Se nuclear magnetic resonance data show an abrupt change in the relaxation rate during this transition. The subsequent transition occurs at a temperature several degrees below and is also accompanied by anomalies in the electronic properties. Thus, for the phase diagram of Fe(Se,Te), as well as for the phase diagrams of FeSe under pressure, there is a region where structural transitions occur in two consecutive stages. We explain this similarity between the corresponding phase diagrams by the same deformation of the iron coordination environment in Fe(Se,Te) compounds and in FeSe under pressure. Our findings provide new and significant information on the phase diagram of Fe(Se,Te) compounds, and in particular, suggest the existence of specific orbital or structural instability in the basic structural element of iron-based superconductors.