An X-ray, Mössbauer and magnetization investigation of hexagonal FeSe

X-ray diffraction (XRD), Mössbauer spectroscopy (MS), and magnetization (SQUID) data are reported for a “pyrrhotite-like” compound (∂-FeSe), belonging to the central portion of the FeSe system. The ∂-phase(s) extends compositionally over a range of around Feo.94Se-Feo.86Se at 870 K, but narrows around Fe7Se8 at room temperature. Lattice parameters for the primitive hexagonal cell decrease regularly with increasing vacancy concentration x in Fe1-.xSe; c = 5.97 — 5.87 A, a = 3.65 — 3.62 A. An observed regular decrease in cell axes ratio, c/a = 1.644 — 0.145 x, is described as due to increased ordering with increased vacancy concentration. The disordered high-temperature 1 c structure is maintained at room temperature for vacancy-poor compositions, while superstructures, 3c and 4c, form for vacancy-rich compositions. Using a bond model for the magnetic coupling, MS spectra for ordered 3c and 4c superstructures can be well fitted assuming three well-defined surroundings, while those for disordered lc show a distribution in surroundings. A spin direction change, from the c-plane towards the c-axis with decrease in temperature, is sharp at 130 K for 3c, extended and incomplete for 4c (~220 — 120 K), and again rather sharp for lc at ∽ 220 K. It is suggested that the spin flip at 130 K for 3c, which is also visible (but weak) in SQUID measurements of lc samples, is indicative of an element of short range order in the disordered structure.