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

Abstract

${\mathrm{Sr}}_{1\ensuremath{-}x}{X}_{x}{\mathrm{FBiS}}_{2}$ $(X:\text{rare earth})$ 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 (${T}_{c}$). We clarified the monoclinic crystal structure ($P{2}_{1}/m$) in the high-pressure phase which is the same as the high-pressure phase of $X{\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}\mathrm{Bi}{Y}_{2}$ ($Y$: chalcogen) systems. We performed high-resolution x-ray absorption spectroscopy (XAS) to study the electronic structure of ${\mathrm{Sr}}_{1\ensuremath{-}x}{X}_{x}{\mathrm{FBiS}}_{2}$ systematically, indicating the Bi charge state of nearly $3+$. The pressure dependence of the XAS spectra of ${\mathrm{Sr}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2}$ and ${\mathrm{Sr}}_{0.5}{\mathrm{Nd}}_{0.5}{\mathrm{FBiS}}_{2}$ 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 $d$ (${t}_{2g}$) 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 ${T}_{c}$ was observed.