Mössbauer study of (Fe1-xNix)7Se8.

(Fe 1+x Ni x ) x Se 8 (x=0.02,0.05,0.08) has been studied with Mossbauer spectroscopy and x-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while Fe 7 Se 8 has a hexagonal structure. Abrupt change of quadrupole shifts near 122 K suggests that the spin-rotation transition proceeds abruptly, in contrast with the gradual transition reported for Fe 7 Se 8 with a triclinic superstructure. The iron ions at all four sites are found to be in a highly covalent ferrous state. Both Neel and Debye temperatures are found to decrease with increasing nickel concentration