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 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. 
V. Svitlyk, M. Raba, V. Dmitriev, P. Rodiere, P. Toulemonde, D.   Chernyshov, M. Mezouar
cond-mat.supr-con