Crystallographic and Mössbauer study of zinc blende type FeS

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Abstract
Until a few years ago two main phases of FeS were known : a hexagonal phase which is antiferromagnetic and semi metallic, and a tetragonal phase which is non magnetic. We present here the results of a crystallographic and Mössbauer study of the new cubic zinc blende type phase which was discovered recently. This phase is unstable at room temperature. When the temperature is decreased it undergoes a first order crystallographic transition (cubic → orthorhombic) at about 234 K. The Mössbauer effect has shown that at all temperatures the new phase is apparently ionic, with high spin Fe++ ions. Above 234 K it is paramagnetic and below 234 K it exhibits a first order magnetic transition to an ordered collinear phase in which the magnetic moments are all parallel to one of the axes of the orthorhombic cell. Various mechanisms have been examined in order to interpret the first order magnetocrystalline transition at 234 K : standard exchange magnetostriction ; Jahn-Teller effect involving the Γ3 orbital doublet, in association with exchange magnétostriction or generalized exchange and so on, but we could not arrive at a definite conclusion. As expected the volume per FeS formula increases in going from the semi metallic hexagonal phase to the non magnetic tetragonal phase, and from the tetragonal phase to the ionic high spin cubic phase. The properties of the tetragonal phase, which had been interpreted as covalent, could perhaps also be described in terms of low spin ferrous ions (strong crystalline field case)

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