Effects of vanadium impurities on the spin-rotation transition of iron selenide “4c” Fe7Se8

Abstract (Fe 0.92 V 0.08 ) 7 Se 8 has been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal structure is found to be a triclinic superstructure of the NiAs structure. Abrupt change of quadrupole shifts at 123 K suggests that the spin-rotation transition proceeds abruptly in contrast with the gradual transition reported for Fe 7 Se 8 with the same triclinic superstructure. The iron ions at all four sites are found to be in a highly covalent ferrous state. Neel and Debye temperatures are found to be 420 ± 2 K and 232 ± K, respectively.