Pressure-induced effects on the structure of the FeSe superconductor

A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P4/nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T=50 K) and tetragonal (T  =190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a Bulk modulus ≈31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the cc-axis. Key bond angles of the SeFe4 pyramids and FeSe4 tetrahedra are also determined as a function of pressure.