抄録
Iron vacancies in Fe7Se8 are confined to the alternate iron layers parallel to the c plane of the fundamental lattice with the NiAs structure. The origin of the ferrimagnetisni of Fe7Se8 (Tc=187°C) may be attributed to this ordered arrangement similarly to the case of Fe7Se8. There are two types of the superstructure: low- and high-temperature ones. The unit cell dimensions and the lattice symmetries of these structures are as follows:
low-temperature type: A=\sqrt3B, B=2a, C=4c, triclinic,
high-temperature type: A=2a, C=3c, hexagonal,
where the capital and small letters designate the super- and fundamental lattices, respectively. The transition from the triclinic structure to the hexagonal one, which means a rearrangement of the iron vacancies, begins at 240°C, proceeds gradually and finishes at 298°C.
