Neutron spectroscopy within the S=5 groud multiplet and low-temperature heat capacity in Fe4 magnetic cluster

The transitions within the S=5 ground state of the tetrairon(III) molecular cluster Fe4(OCH3)6(dpm)6 (Hdpm = dipivaloylmethane) have been measured by inelastic neutron scattering. The spectra have been interpreted by means of an effective spin Hamiltonian for an isolated S=5 multiplet, taking into account the presence of three different isomers in the compound. It has been shown that for the two dominant isomers the symmetry of the zero-field splitting tensor is not purely axial, but a nonzero rhombic coefficient E is needed to fit the neutron spectra. The results have been discussed in comparison with recent high-field electron paramagnetic resonance data in single crystals. Heat-capacity measurements in the range 2 to 20 K have been performed and shown to be compatible with the neutron results for the zero-field splitting.