Analysis of Far-Infrared Spectra of Antiferromagnetic FeCO3

The transmission spectrum of antiferromagnetic FeC${\mathrm{O}}_3$ has been investigated in the far-infrared region from 20 to 300 ${\mathrm{cm}}^{-1\mathrm{}}$, for temperatures from 4.2 to 70 K and in applied fields up to 90 kOe. Spectra were obtained from natural single crystals and high-purity synthetic powders. At 4.2 K, a sharp electronic transition (magnetic dipole) was observed at 112.5 ${\mathrm{cm}}^{-1\mathrm{}}$, exhibiting parallel and perpendicular splitting factors of 6.6 and O, respectively. Its oscillator strength is $f=2.1\mathrm{}\times {10}^{-8\mathrm{}}$. An additional weak electronic line is observed at 160 ${\mathrm{cm}}^{-1\mathrm{}}$. The details of these transitions and electronic Raman transitions observed by others are successfully described by the ${\mathrm{Fe}}^{2+}$ single-ion Hamilitonian $\mathcal{H}=R\lambda \overrightarrow{\mathrm{L}}·\overrightarrow{\mathrm{S}}+R^2\zeta (L_z^2-\frac{2}{3})+\mathcal{J}{S}_z$, where $\lambda$, $\zeta$, and $\mathcal{J}$ are the spin-orbit, trigonal-field, and Ising-molecular-field parameters, respectively, and $R$ is an "orbital-reduction" factor. The analysis yields values for these parameters of $R\lambda =105\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $-R^2\zeta =1506\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $R\approx 1$, and $\mathcal{J}=19\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The observed far-infrared transitions are identified as being within the low ${}_{}{}^5{}_{}{}^{}E$ spin-orbit manifold from the ground state ($M_J=-1\mathrm{}$) to the two components of the $M_J=0\mathrm{}$ doublet. The temperature dependence of the 112.5-${\mathrm{cm}}^{-1\mathrm{}}$ transition is studied from 4.2 to 70 K, and its intensity is observed to drop sharply as the temperature is raised to the vicinity of $T_N=38.4\mathrm{}$ K. This behavior is compared to that of an Ising spin system. In addition, three phonons are observed at 186, 201, and 224 ${\mathrm{cm}}^{-1\mathrm{}}$ and are identified as external