Crystal and electronic structures of BiS2-based compounds Sr0.5X0.5FBiS2 (X = rare earth) under pressure: Correlation with the change in the superconductivity from unconventional to conventional

Sr1-xXxFBiS2 (X : rare earth) is considered to show a pressure-induced structural transition around 1 GPa where the superconductivity changes from unconventional in the low-pressure phase to conventional in the high-pressure phase with increasing the superconducting transition temperature (Tc). We clarified the mono-clinic crystal structure (P21/m) in the high-pressure phase which is the same as the high-pressure phase of XO1-xFxBiY2 (Y : chalcogen) systems. We performed high-resolution x-ray absorption spectroscopy (XAS) to study the electronic structure of Sr1-xXxFBiS2 systematically, indicating the Bi charge state of nearly 3+. The pressure dependence of the XAS spectra of Sr0.5La0.5FBiS2 and Sr0.5Nd0.5FBiS2 indicate a large change in the electronic structure around the structural transition pressure, which corresponded to the transition from semiconductorlike to more metallic states. The increase of the empty states of Bi s and d (t2g) bands above the Fermi level was observed in the high-pressure phase. Density functional theory calculations showed that the density of the states at the Fermi level did not show a significant change in the high-pressure phase, where higher Tc was observed.