Structure of the ferrocene carboxaldehyde mesophase by X-ray diffraction and Mössbauer spectroscopy

Abstract

Ferrocene carboxaldehyde is unusual in that whilst its molecular shape is of low symmetry it shows a mesophase between 44°C and the m.p. (123°C). The nature of this phase has been investigated by X-ray diffraction and Mössbauer spectroscopy. It is shown to be an orientationally disordered molecular crystal, with a simple f.c.c. lattice (a= 999 ± 2 pm). The molecules are, on average, randomly oriented about their centres of mass C (r_CFe= 46 pm). Short range correlations in molecular arrangement are accounted for by a Gaussian distribution of molecular centres with 〈U²_x〉^½= 69 pm. In contrast the r.m.s. displacement of the Fe in the room temperature phase is ∼ 20 pm as for ferrocene.