High-pressure X-ray absorption and diffraction study of the self-doped superconductor EuFBiS2

The BiS2-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the EuFBiS2 compound, showing a superconducting transition temperature Tc∼0.3 K at ambient pressure. Upon increasing external pressure, Tc goes through a large amplification, accompanied by a structural phase transition (SPT) from tetragonal to monoclinic symmetry. Here, we use a combination of Eu L3-edge X-ray absorption spectroscopy and synchrotron X-ray diffraction to unveil the evolution of the Eu valence and lattice symmetry under high pressure. We find that the average Eu valence increases gradually with pressure, exhibiting a pressure plateau near the SPT, at which the Tc increases sharply. Since in EuFBiS2 the charge carriers are introduced via self-doping induced by the mixed valence of the Eu ions, our findings clearly indicate that the role of the charge doping is marginal in the Tc enhancement. On the other hand, the structural distortions, taking place at the SPT, play a central role in enhancing the superconducting properties of the EuFBiS2 system.