  A high-frequency (95 GHz) EPR study is reported on single crystals of the
planar tetranuclear complex Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane),
which has been previously shown to present typical single-molecule magnet
behaviour. The spectra, all originating from the S = 5 ground state, possess
quasi-axial symmetry along the normal to the plane defined by the four Fe(III)
ions. The measured spectra are shown to belong to three different structural
variations of the compound, resulting from disorder in the ligands around two
of the Fe(III) ions. Accurate values could be obtained for the second- and
fourth-order crystal field parameters related to the parallel EPR-spectra,
while the other parameters could be determined only for the dominant species.
The separation between individual lines is decreasing and vanishing with
increasing temperature. This effect is attributed to the contribution of fast
relaxing excited states, whose population is varying with temperature.
