Polarized absorption and magnetic circular dichroism of the FeCl₄^2- ion in a tetragonally distorted tetrahedral site

Abstract

Pure single crystals of Cs₃FeCl₅ have been prepared from the melt and characterized by Raman, polarized absorption and Magnetic Circular Dichroism (MCD) spectroscopies. Most of the observed features of the spectra can be understood on the basis of a single ion spin-orbit coupling mechanism. Relative transition probabilities have been derived using that scheme for the various possible ⁵ E →³Γ transitions. Our spectroscopic data indicate that, as for CoCl₄ ^2- in Cs₃CoCl₅, the geometry of the FeCl₄ ^2- ion in Cs₃FeCl₅ is a tetrahedron elongated along one of the C ₄ axes. The ground state is ⁵ Eε with the M_S = ± 1 and M_S = ± 2 components at 1·2 cm^-1 and 4 cm^-1 respectively; the ⁵ Eϑ manifold lies around 25 cm^-1. Most of the major bands are assigned on the basis of the sign of the MCD, in the light of a crystal field model which proves reasonably satisfactory. The possible occurrence of an exchange induced dipole mechanism is thought to be responsible for some disagreement between experimental results and our predictions for the single ion mechanism.