Intrinsic Phase Diagram of Superconductivity in the BiCh2-based System Without In-plane Disorder

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.