  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.
