$\alpha \text{-FeSe}$ 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 $\sim 7\text{GPa}$ before decreasing to 6 K upon further compression to $\sim 14\text{GPa}$. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $\alpha \text{-FeSe}$, revealing an extremely soft solid with a bulk modulus, $K_0=30.7\left(1.1\right)\text{GPa}$ and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed $\beta$ polymorph above $\sim 9\text{GPa}$. The nonmonotonic $T_c\left(P\right)$ 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.

The newly discovered superconductor FeSe_1−x (x≈ 0.08, T_c ^onset≈ 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure—the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO_1−xF_x (RE = rare earth) superconductors.