The effect of RE substitution in layered REO_0.5F_0.5BiS₂: Chemical pressure, local disorder and superconductivity

We have studied the effect of RE substitution on the structure and the local atomic disorder in REO_0.5F_0.5BiS₂ (RE = rare-earth) to understand their correlation with the bulk superconductivity in these materials. The mean RE size, affecting the chemical pressure, has been varied in two series namely Ce_1-xNd_xO_0.5F_0.5BiS₂ and Nd_1-ySm_yO_0.5F_0.5BiS₂. The lattice parameters evolve anomalously, showing an anisotropic shrinkage (elongation) of the c-axis (a-axis) to an isotropic expansion of the lattice with increasing mean RE size. The Bi L₃-edge extended X-ray absorption fine structure (EXAFS) measurements are performed to investigate local displacements in the BiS₂ lattice, revealing a large disorder and a sharp boundary between the Ce-containing and Sm-containing series with a distinct local structure. The results suggest that the bulk superconductivity in REO_0.5F_0.5BiS₂ is correlated with anomalous atomic displacements in the Bi-S1 network, likely to be a combined effect of active Bi 6s electronic states and a possible Jahn-Teller-like instability of the Bi 6p_x/6p_y electrons.