Mössbauer study of (${\mathrm{Fe}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ni}}_{\mathit{x}}$${)}_7$${\mathrm{Se}}_8$

(${\mathrm{Fe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${)}_7$ ${\mathrm{Se}}_8$ (x=0.02,0.05,0.08) has been studied with Mössbauer spectroscopy and x-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure while ${\mathrm{Fe}}_7$ ${\mathrm{Se}}_8$ 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 ${\mathrm{Fe}}_7$ ${\mathrm{Se}}_8$ with a triclinic superstructure. The iron ions at all four sites are found to be in a highly covalent ferrous state. Both Néel and Debye temperatures are found to decrease with increasing nickel concentration.