Complex biphase nature of the superconducting dome of the FeSe phase diagram

Single crystal synchrotron x-ray diffraction as a function of temperature and pressure has revealed a complex biphase mixture in superconducting FeSe. Based on our experimental results, we construct a phase diagram where structural behavior and superconducting properties of FeSe are found to be correlated. We show that below 6 GPa, where pressure promotes the superconducting critical temperature, the FeSe structure is composed of two-dimensional layers of edge-shared $\mathrm{FeS}{\mathrm{e}}₄$ tetrahedra, while above 6 GPa, the superconductivity is strongly suppressed on formation of a new orthorhombic polymorph characterized by a three-dimensional network of face sharing $\mathrm{FeS}{\mathrm{e}}₆$ octahedra. Therefore, changes in topology and connectivity of the FeSe structure are found to be detrimental to superconductivity. The previously controversial crystal structure of the high-pressure polymorph of FeSe was unambiguously determined in the present paper. High-pressure FeSe adopts an orthorhombic MnP-type structure (Pnma) which corresponds to a slightly distorted hexagonal NiAs-type arrangement ($P{6}₃/mmc$). The structural transformation from the low- to high-pressure FeSe polymorph is first order in nature and is manifested as antiparallel displacements within the Fe and Se sublattices.