The local structure of self-doped BiS2-based layered systems as a function of temperature

Abstract

We have studied the local structure of layered Eu(La,Ce)FBiS₂ compounds by Bi L₃-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. We find that the BiS₂ sub-lattice is largely distorted in EuFBiS₂, characterized by two different in-plane Bi–S1 distances. The distortion is marginally affected by partial substitutions of Ce (Eu_0.5Ce_0.5FBiS₂) and La (Eu_0.5La_0.5FBiS₂). The temperature dependence of the local structure distortion reveals an indication of possible charge density wave like instability in the pristine self-doped EuFBiS₂ and Ce substituted Eu_0.5Ce_0.5FBiS₂ while it is suppressed in La substituted Eu_0.5La_0.5FBiS₂. In compounds with higher superconducting transition temperature, the axial Bi–S2 bond distance is elongated and the related bond stiffness decreased, suggesting some important role of this in the charge transfer mechanism for self-doping in the active BiS₂-layer. In-plane Bi–S1 distances are generally softer than the axial Bi–S2 distance and they suffer further softening by the substitutions. The results are discussed in relation to an important role of the Bi defect chemistry driven asymmetric local environment in the physical properties of these materials.