Mössbauer and X-ray studies of (Fe1−xMx)7Se8, (M = Ni, Co)

Abstract

(Fe1−xNix)7Se8 and (Fe1−xCox)7Se8 (x = 0.02, 0.05, 0.08) have been studied with Mössbauer spectroscopy and X-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while Fe7Se8 has a hexagonal structure.Abrupt changes of quadrupole shifts near 122 K for (Fe1−xNix)7Se8 and 124 K for (Fe1−xCox)7Se8 suggests that the spin-rotation transition proceeds abruptly, in contrast with the gradual transition reported for Fe7Se8 with a triclinic superstructure. Isomer shifts indicate a ferrous character for the iron ions at all four sites. Both Néel and Debye temperatures are found to decrease with increasing nickel and cobalt concentration.