Structural transition at 225 K of the trinuclear Fe(III) heptanoate [Fe3O(O2CC6H13)6(H2O)3]NO3

The trinuclear iron (III) heptanoate [Fe3O(O2CC6H13)6(H2O)3]NO3 compound is synthesized and studied by single crystal X-ray diffraction as a function of the temperature, also by Fe57 Mössbauer spectroscopy, electrochemical and magnetic measurements. Its thermal behavior is determined by DSC, TGA and X-ray diffraction. The compound undergoes a structural phase transition at Ts = 225 K giving a superstructure of order two (above Ts: P −1, a=11.1393(2)Å, b=14.2757(3)Å, c=18.2088(4)Å, α=80.607(1)°, β=84.175(1)°, γ=83.464(1)°; below Ts: P −1, a=11.0122(2)Å, b=14.2723(3)Å, c=35.5222(7)Å, α=94.138(1)°, β=97.136(1)°, γ=96.779(1)°). The origin of the transition is interpreted as being due to a 60° rotation of the nitrate groups of one sheet out of two of the lamellar structure. Analysis of the Fe57 Mössbauer spectra as a function of the temperature does not show electronic transition such as valence trapped phenoma below Ts. The compound has a paramagnetic behavior in the temperature range 300-2 K. A Heisenberg model of interaction based on triangular Fe(III) units of quantum S=5/2 spins is used for simulating the variation of the susceptibility between room temperature and 2 K.