Institut für Festkörperphysik




Polyanionc infinite [Fe2S3]2- double chains formed by edge-sharing [FeS4] tetrahedron are characteristic for the crystal structure of the thioferrate BaFe2S3 (space group Cmcm). The Ba2+ cations are interspersed between these chains. Crystallographically only one Fe site is present, which should be divalent. Mössbauer investigations reveals, however, an isomer shift pointing towards a mixture of di- and trivalent Fe atoms. For isostructural KFe2S3, with cation charge +1, for Fe a valence state of 2.5 is expected, which may be realized by a random distribution of Fe2+ and Fe3+ at the same crystallographic site. Quite unique for partially ionic compounds, BaFe2S3 and KFe2S3 form a continuous solid solution. This offers the possibility to study the influence of the cation distribution on the not well established character of the bonding between Fe and S within the tetrahedron.

BaFe2X, X = S, Se
A strange behaviour is obtained if BaFe2S3S is compared with BaFe2S3. Although the crystal structure does not change, strongly different magnetic behaviour is present. From 57Fe Mössbauer investigations both samples are characterized as mixed valence compounds, Whereas at 4.2 K for the sulphur compound only one hyperfine split spectrum is present, a field distribution with approximately a factor of two larger mean hyperfine field is observed for the Se sample. Reason for this may be the different local symmetry around the Fe atom in the two structure types. Whereas in BaFe2S3 two different S atoms are in the first neighbour shell, Fe in BaFe2S3 is surrounded by three different Se atoms. This might explain the larger quadrupole splitting for the Se sample. The mean bond lengths Fe-Fe and Fe-S(Se) are approximately 3 and 10% larger in the Se compound.

Fig.1: Orthorhombic crystal structure
(a ~ 8.8 Å, b ~ 11.2 Å, c ~ 5.3 Å).
Contact: M. Reissner, W. Steiner