Pressure-Driven Structural and Electronic Evolution in CsFe₄₋_δSe₄

FeSe-based materials exhibit rich correlated electronic behavior, including orbital-selective Mott states and unconventional superconductivity. The recently identified compound CsFe₄₋δSe₄ has been proposed to host a Mott insulating ground state [1,2], with a structure closely related to alkali-intercalated FeSe superconductors [3]. Although superconductivity emerges under pressure, the mechanism linking the collapse of the insulating state to superconductivity remains unclear.

Here, we combine synchrotron X-ray diffraction, Raman spectroscopy, X-ray absorption near-edge structure (XANES), electrical transport, and first-principles calculations to investigate CsFe₄₋δSe₄ under compression. X-ray diffraction reveals a pressure-induced structural transition from an orthorhombic (Cmmm) to a monoclinic (Cm) phase near 10 GPa. Raman spectra corroborate this transition through pronounced changes in phonon mode intensities. XANES measurements indicate Fe–Se charge transfer and enhanced d-band itinerancy with increasing pressure. At higher pressures, we observe superconducting signatures up to ~10 K at 55.2 GPa.

These results illuminate how structural distortion, charge redistribution, and evolving electronic correlations cooperate to drive superconductivity from a Mott insulating background in CsFe₄₋δSe₄.