Transition-metal dichalcogenides from disintercalation processes. Crystal structure determination and Mossbauer study of Li₂FeS₂ and its disintercalates Li_xFeS₂ (0.2⩽x⩽2)

In agreement with its hexagonal subcell parameters (a=b=3.908(2) AA and c=6.279(4) AA) and c/a ratio of 1.601, the Li₂FeS₂ structure was found to be constructed from a hexagonal close packing of S^2- anions with tetrahedral iron ions. Different filling of the two (FeS₄) sites was found to be responsible for the 2a*2b*c supercell. Lithium ions are scattered on tetrahedral and octahedral sites, some of the latter remaining empty because of intercationic Coulombic repulsions. Mossbauer study of Li_xFeS₂ disintercalates (0.2<or=x<or=2) shows the occurrence of oxidation of iron(II) into iron(III) and a sudden change from tetrahedral to octahedral coordination in accord with the break in the equilibrium discharge curve at Li_1.5FeS₂ of a Li_xFeS₂/Li electrochemical cell. The occurrence of a new iron disulphide FeS₂ (Fe³⁺S^2-(S₂)^2-₁2/), as characterised by previous infrared analyses, is confirmed by its Mossbauer characteristics showing two distinct iron(III) sites different from that of FeS₂ of the pyrite or marcassite type. The oxidoreduction process of the disintercalations and intercalations involving iron and sulphur as redox centres is better understood as well as their actions on the cationic structural alterations.