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

alpha-FeSe with the PbO structure is a key member of the family of high-T-c 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 T-c which attains a broad maximum of 37 K at similar to 7 GPa before decreasing to 6 K upon further compression to similar to 14 GPa. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of alpha-FeSe, revealing an extremely soft solid with a bulk modulus, K-0=30.7(1.1) GPa and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed beta polymorph above similar to 9 GPa. The nonmonotonic T-c(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.