Magnetic anisotropy and electronic structure of gillespite, a mineral containing planar, four-co-ordinate, high-spin iron(II)

Abstract

The single-crystal magnetic anisotropy of gillespite, a rare mineral containing Fe^II in a square-planar ligand geometry, is reported over the range 4.2–298 K. Good agreement is obtained between the calculated and observed magnetic parameters using a crystal-field model assuming a ⁵A_1g ground state and with excited state energies compatible with the electronic spectrum. However, the value of the orbital-reduction parameter k is considerably lower than expected. An analysis of the d-orbital energies in gillespite and the isomorphous copper analogue egyptian blue using the angular-overlap model is presented. This suggests the presence of an abnormally strong π-bonding interaction, although more reasonable ligand-bonding parameters are obtained if it is assumed that the a_1g(d_z²) orbital is depressed in energy by ca. 5 000 cm^–1 by configuration interaction with the a_1g(4s) orbital in these compounds. It is shown that the electronic spectrum of the high-pressure form of gillespite is consistent with a flattened tetrahedral ligand geometry about the Fe^II, but it is proposed that the distortion from planarity is probably considerably greater than that suggested as a result of a previous X-ray crystal-structure determination.