SEMICONDUCTOR TO TRANSITION IN FeS

Summarised here are the conclusions of an investigation by differential thermal analysis, magnetic susceptibility, Mossbauer spectroscopy, electrical conductivity and thermoelectric power on single crystal Fe0.996S. There is an increase in conductivity both parallel and perpendicular to the c-axis by a factor 102 at Ta (420 K). At low temperature the I. S. is characteristic of a non-metal but its temperature dependence appears to be anomalous. Quadrupole hyperfine parameters give some indication of the band structure in the low-temperature (2C) phase. Mossbauer magnetic-hyperfine field data together with the susceptibility results suggest that the magnetic moment does not change significantly at Ta. The Debye temperature, calculated from the Mossbauer fractions, drops significantly at the transition, indicating a lattice contribution to the entropy of transition. We conclude that FeS is a localised-spin antiferromagnet and narrow-band-gap semiconductor below Ta, and a localized-spin antiferromagnet and a not-well-behaved metal above Ta. If the increase in conductivity at Ta arises from delocalization of the 6th 8-spin electron, the itinerant electron appears to retain a moment.