Abstract
We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.
Highlights
Layered materials have been a subject of intense research during these decades due to possibility of control and manipulation of their physical properties to obtain desired functions
We have studied two samples (LaO0.77F0.23BiS2 and NdO0.71F0.29BiS2) in which the doping is introduced by substitution while three samples are self-doped (EuFBiS2, CeOBiS2 and PrOBiS2)
Self-doped systems (EuFBiS2, PrOBiS2, CeOBiS2) show relatively damped oscillations for k ≥ 10 Å−1, likely to be related with mixed valence of the RE atoms since the higher k-oscillations are sensitive to the heavier atoms
Summary
Layered materials have been a subject of intense research during these decades due to possibility of control and manipulation of their physical properties to obtain desired functions. The discovery of high Tc superconductivity in layered copper oxides [1] had been one of the major breakthroughs and since a large number of new layered materials have been found to show unconventional superconductivity. The research in the field was further fuelled by the discovery of superconductivity in the layered iron arsenides [2] resulting in several new layered systems. BiS2-based systems [3,4] have received increasing attention during the last decade both for their superconducting and thermoelectric properties, leading to numerous theoretical and experimental studies to understand their functional properties [5,6,7,8]. The highest superconducting transition temperature Tc ∼10.5 K has been found in high pressure grown LaO1−xFxBiS2 system
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