In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure

BiCh₂-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh₂-based superconductor family, specifically, optimally doped Ce_1-xNd_xO_0.5F_0.5BiS₂ and LaO_0.5F_0.5Bi(S_1-ySe_y)₂. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO_0.5F_0.5BiCh₂ superconductors can be determined from the in-plane chemical pressure.