Mossbauer and X-ray study of (Fe/sub 1-x/Ni/sub x/)/sub 7/Se/sub 8/ and (Fe/sub 1-x/Co/sub x/)/sub 7/Se/sub 8/.

Mossbauer spectroscopy and X-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while Fe/sub 7/Se/sub 8/ has a hexagonal structure. Changes of quadrupole shifts near 122 K for (Fe/sub 1-x/Ni/sub x/)/sub 7/Se/sub 8/ and 124 K for (Fe/sub 1-x/Co/sub x/)/sub 7/Se/sub 8/ indicate that the spin-rotation transition proceeds abruptly, in contrast to the gradual transition reported for Fe/sub 7/Se/sub 8/ with a triclinic superstructure. Isomer shifts indicate a ferrous character for the iron ions at all four sites. The Neel temperature is found to decrease with increasing nickel and cobalt concentration. [ABSTRACT FROM PUBLISHER]