Structural and optical investigations of the iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ under high pressure

Iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ has been investigated under high pressure using synchrotron based x-ray diffraction and mid-infrared reflectance measurements at room temperature. Pressure dependence of the superconducting transition temperature (T$_c$) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. Although the high pressure orthorhombic phase ($\textit{Pbnm}$) starts emerging at 4 GPa, structural transition becomes clearly observable above 10 GPa. A strong correlation is observed between the Fe(Se,Te)$_{4}$ tetrahedral deformation in the tetragonal phase ($\textit{P4/nmm}$) and the sharp rise of T$_c$ up to $\sim$4 GPa, above which T$_c$ is found to be almost pressure independent at least up to 10 GPa. A subtle structural modification of the tetragonal phase is noticed above 10 GPa, suggesting a structural transition with possible Fe$^{2+}$ spin-state transition. The evolution with pressure of the optical conductivity shows that the Drude term increases systematically with pressure up to 5.4 GPa, indicating the evolution of the tetragonal phase towards a conventional metallic state. At further higher pressures the Drude term reduces drastically implying a poor metallic character of the high pressure orthorhombic phase. Our results suggest that occurrence of large onset T$_c$ above 4 GPa is due to a systematic increase of high pressure orthorhombic phase fraction.

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