$\alpha \text{-FeSe}$ 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.

• Received 13 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.064506