Magnetisation and Mossbauer studies of (Fe1-xMex)(2)SiS4, x <= 0.25 and Me= Co or Ni

The olivine related compound (Fe1-xMex)(2)SiS4, Me = Co or Ni, has been stu died by magnetisation (SQUID) measurements and Mossbauer spectroscopy There are two crystallographic metal positions in the structure, M1 having an in version center and M2 possessing a mirror plane. Fe2SiS4 becomes antiferrom agnetic (AFM) at 126 K and additionally experiences a ferrimagnetic (FiM) t ransition at 32 K that gives rise to a net spontaneous magnetic moment of a bout 0.15 mu(B)/iron atom. The Neel temperature increases slightly for Co a nd substantially for Ni substitution whereas the FiM transition temperature is strongly suppressed in both cases. The hyperfine held on M2 is up to 8 times smaller than on M1 in the AFM phase, while in the RM state the hyperf ine fields of the two sites become of a more comparable magnitude. A simila r anomalous behaviour of the AFM hyperfine fields remains in the samples wi th Co or Ni substitution. We could also definitely make the assignment that Fe-ions in M2 sites give higher centroid shift values and a lower magnitud e of the quadrupole splitting than Fe in M1 sites in the Mossbauer spectra. This result also ascertains that it is the M1 sites that are preferentiall y occupied by Co and Ni.