Role of the local structure in superconductivity of LaO 0.5 F 0.5 BiS 2− x Se x system

We have studied the local structure of LaO 0.5 F 0.5 BiS 2− x Se x by Bi L 1 -edge extended x-ray absorption fine structure (EXAFS). We find a significant effect of Se substitution on the local atomic correlations with a gradual elongation of average in-plane Bi-S bondlength. The associated mean square relative displacement, measuring average local distortions in the BiS 2 plane, hardly shows any change for small Se substitution, but decreases significantly for x ⩾ 0.6 . The Se substitution appears to suppress the local distortions within the BiS 2 plane that may optimize in-plane orbital hybridization and hence the superconductivity. The results suggest that the local structure of the BiS 2 -layer is one of the key ingredients to control the physical properties of the BiS 2 -based dichalcogenides.