STRUCTURAL ASPECTS OF THE ALPHA-TRANSITION IN OFF-STOICHIOMETRIC FE1-XS CRYSTALS .2.

Study of the α transition in the Fe1-xS system (NiAs-type solid solution) is completed by structural analysis versus temperature of the cation-deficient crystals Fe0.985S and Fe0.975S. The influence ofx andT on the disorder phenomena and on the gradual change of the cationic lattice is analyzed in the LT hexagonal√3a,2c phase (a andc are NiAs-type substructure lattice constants). The hypothesis of the existence of an S-1 impurity level in the band gap of FeS, originating from the formation of covalent pairs (S{single bond}S)2- around each vacancy, leads to a model of conductivity by polarons. This accounts for the decrease withx in the transition energy. The transition is suppressed forx = 0.05, which corresponds to the critical polaron concentration beyond which the polaron gas becomes degenerate. © 1990 Academic Press, Inc.