MOSSBAUER STUDY OF 3C SUPERSTRUCTURE OF THE FERRIMAGNETIC FE7SE8

Fe7Se8 has been studied by Mossbauer spectroscopy and X-ray diffractio n. The crystal structure is found to be a ''3c'' hexagonal superstruct ure of the NiAs structure. It is shown that the Mossbauer results obta ined at temperatures below 60 K are also consistent with the Okazaki's ''3c'' superstructure model. Abrupt change of quadrupole shifts near 130 K suggests that the spin-rotation transition proceeds abruptly. Th e iron ions at all three sites are found to be in a highly covalent fe rrous state. The Noel temperature is measured to be 447 +/- 2 K in fai r agreement with the previously reported value. The Debye temperature is also found to be 248 +/- 5 K.