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.Since the discovery of Bi4O4S3 and REO1-xFxBiS2 (RE: rare earth) superconductors, BiCh2-based (Ch: chalcogen) superconductors have been drawing much attention as a new class of layered superconductors [1][2][3]. Since the crystal structure composed of alternate stacks of the electrically conducting BiCh2 layers and the insulating (blocking) layers resembles those of the Cuprate and the FeAs-based high-transition-temperature (high-Tc) superconductors [4,5], many experimental and theoretical studies have been performed to clarify the superconductivity mechanisms of this system and to increase Tc. However, the mechanisms have not been understood completely. Recently, Morice et al. proposed that a 2 weak-coupling electron-phonon mechanism cannot explain the emerging Tc, as high as 11 K, in the BiCh2-based systems, from first principle calculations [6]. Therefore, full understandings of the basic characteristics of the superconductivity in the BiCh2-based system are crucial.The parent phase of the BiCh2-based superconductor is a band insulator [1,7,8]. On the basis of the calculated band structure, electrons carriers are doped into the bands mainly composed of Bi-6px and Bi-6py components. Electron-doped BiCh2-based compounds are expected to become metallic. Indeed, superconductivity is experimentally observed in electron-doped compounds [1][2][3]. However, the real situations are not simple as expected from the band structure. Although the superconductivity in BiCh2-based compounds is emerged by carrier doping, metallic transport is sometimes absent, and weakly localized behavior (semiconducting-like behavior) is observed in electrical resistivity measurements; a good example would be optimally doped LaO0.5F0.5BiS2. In semiconducting-like samples of LaO0.5F0.5BiS2, bulk superconductivity is not observed, while weak (filamentary) superconductivity is observed. To induce bulk superconductivity in the LaO0.5F0.5BiS2 system, external pressure effects [9][10][11][12][13][14][15][16] and/or element substitution at the La site [17][18][19][20], which optimize the crystal structure, are available. Namely, both electron carrier doping and crystal structure optimization are required for the e...
