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 Fe-57 Mossbauer 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) angstrom, b = 14.2757(3) angstrom, c = 18.2088(4) angstrom, alpha = 80.607 (1)degrees, beta = 84.175 (1)', gamma = 83.464(1)degrees; below Ts: P-1, a = 11.0122(2) angstrom, b = 14.2723(3) angstrom, c = 35.5222(7) angstrom, alpha = 94.138(1)degrees, beta = 97.136(1)degrees, gamma = 96.779(1)degrees). The origin of the transition is interpreted as being due to a 60 degrees rotation of the nitrate groups of one sheet out of two of the lamellar structure. Analysis of the Fe-57 Mossbauer 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. (c) 2005 Elsevier SAS. All rights reserved.