Mossbauer study of Fe7Se8-xSx

Effects of interatomic separation on superexchange interactions, interatomi c binding, and spin-rotation transition in Fe7Se8-xSx have been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal structure is foun d to be "3c" hexagonal superstructure of the NiAs structure, and lattice co nstants A and C decrease linearly with increasing sulphur concentration. As interatomic distance decreases, Debye temperature, Neel temperature, and m agnetic hyperfine field increase monotonically. When the number of sulfur i ons replacing selenium ions increases from 1 to 7, the interatomic distance decreases by 4.4%, and Debye temperature, Neel temperature, and magnetic h yperfine field at 15 K increase by 12%, 10%, and 6%, respectively. While th e spin rotation near 130 K persists for Fe7Se7S, it disappears in Fe7Se8-xS x (x greater than or equal to 2). The iron ions at all three sites are foun d to be in a highly covalent ferrous state.