Pressure evolution of the low-temperature crystal structure and bonding of the superconductor FeSe (Tc =37 K)

α-FeSe with the PbO structure is a key member of the family of high- Tc iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe4 tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that the application of hydrostatic pressure first rapidly increases Tc which attains a broad maximum of 37 K at ∼7 GPa before decreasing to 6 K upon further compression to ∼14 GPa. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of α-FeSe, revealing an extremely soft solid with a bulk modulus, K0 =30.7 (1.1) GPa and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed β polymorph above ∼9 GPa. The nonmonotonic Tc (P) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties.