Two-center transitions in the antiferromagnetic salt FeCO3

The optical-absorption spectrum of the antiferromagnetic salt FeC${\mathrm{O}}_3$ has been measured. As many as 34 bands have been observed. In order to give a reasonable explanation of the observed spectrum, an ab initio—like calculation including a trigonal crystal field has been made. Three stronger bands at 24 390, 24 875, and 25 126 ${\mathrm{cm}}^{-1\mathrm{}}$ are interpreted in terms of two-center transitions of ${\mathrm{Fe}}^{2+}$-ion pairs. The essential idea is the application of an analytical approximation of Watson's self-consistent-field procedure to obtain the spin-orbit interaction and the electrostatic parameters for the isolated ions. Crystal-field energy levels are then calculated using the nearest-neighbor point-ion approximation. Despite these approximations, the results are quite good.