  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 2D layers of edge-shared FeSe4
tetrahedra, while above 6 GPa the superconductivity is strongly suppressed on
formation of a new orthorhombic polymorph characterized by a 3D network of face
sharing FeSe6 octahedra. Therefore changes in topology and connectivity of the
FeSe structure are found to be detrimental for superconductivity to exist. This
previously controversial crystal structure of the high pressure polymorph of
FeSe was also unambiguously determined. High pressure FeSe adopts an
orthorhombic MnP-type structure (Pnma) which corresponds to a slightly
distorted hexagonal NiAs-type arrangement (P63/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.
