A new isomorph of ferrous chloride tetrahydrate: A 57Fe Mössbauer and X-ray crystallography study

This paper outlines the discovery of a newly characterised isomorph of ferrous chloride tetrahydrate, Fe(H2O)6·FeCl4(H2O)2, which was initially identified by X-ray crystallography and confirmed by Mössbauer spectroscopy. The X-ray analysis identified the space group as P21/c with essentially the same unit cell dimensions as the well-known isomorph, FeCl2·4H2O, except that one edge is doubled due to two discrete [Fe(H2O)6]2+ and [FeCl4(H2O)2]2− species per unit cell. Time-series Mössbauer studies revealed this new isomorph to be unstable upon atmospheric exposure, decaying to the well-known structure over a period of days. Density functional theory calculations support an energetically favourable catalytic interconversion involving adsorbed water. A high-precision redetermination on the FeCl2·4H2O crystal structure, which is also in space group P21/c, is also reported, providing the unit cell parameters: a=5.8765(3) Å, b=7.1100(3) Å, c=8.4892(5) Å and β=111.096(1)°.