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

$\ensuremath{\alpha}\text{-FeSe}$ 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 ${\text{FeSe}}_{4}$ 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 $\ensuremath{\sim}7\text{ }\text{GPa}$ before decreasing to 6 K upon further compression to $\ensuremath{\sim}14\text{ }\text{GPa}$. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $\ensuremath{\alpha}\text{-FeSe}$, revealing an extremely soft solid with a bulk modulus, ${K}_{0}=30.7(1.1)\text{ }\text{GPa}$ and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed $\ensuremath{\beta}$ polymorph above $\ensuremath{\sim}9\text{ }\text{GPa}$. 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.