AN X-RAY, MOSSBAUER AND MAGNETIZATION INVESTIGATION OF HEXAGONAL FESE

X-ray diffraction (XRD), Mossbauer spectroscopy (MS), and magnetizatio n (SQUID) data are reported for a ''pyrrhotite-like'' compound (delta- FeSe), belonging to the central portion of the Fe-Se system. The delta -phase(s) extends compositionally over a range of around Fe0.94Se-Fe0. 86Se at 870 K, but narrows around Fe7Se8 at room temperature. Lattice parameters for the primitive hexagonal cell decrease regularly with in creasing vacancy concentration x in Fe1-xSe; c = 5.97 - 5.87 angstrom, a = 3.65 - 3.62 angstrom. An observed regular decrease in cell axes r atio, c/a = 1.644 - 0.145 x, is described as due to increased ordering with increased vacancy concentration. The disordered high-temperature 1c structure is maintained at room temperature for vacancy-poor compo sitions, while superstructures, 3c and 4c, form for vacancy-rich compo sitions. Using a bond model for the magnetic coupling, MS spectra for ordered 3c and 4c superstructures can be well fitted assuming three we ll-defined surroundings, while those for disordered 1c show a distribu tion in surroundings. A spin direction change, from the c-plane toward s the c-axis with decrease in temperature, is sharp at 130 K for 3c, e xtended and incomplete for 4c (approximately 220 - 120 K), and again r ather sharp for 1c at approximately 220 K. It is suggested that the sp in flip at 130 K for 3c, which is also visible (but weak) in SQUID mea surements of 1c samples, is indicative of an element of short range or der in the disordered structure.