Exchange-split crystal-field levels ofFe2+in FeCO3

Neutron time-of-flight measurements of the low-lying crystal-field excitations have been taken on both a natural single crystal and pure synthetic powder of FeC${\mathrm{O}}_3$ above and below the Néel temperature ($T_N=38.4\mathrm{}$ K). At 4.2 K the principal allowed excitation was measured to be 13.75±0.10 meV, in good agreement with previous neutron and far-infrared measurements. A second less intense excitation was observed at 19.75±0.25 meV. Above $T_N$ a broadened excitation at 11.2 meV was observed, with a small shoulder persisting on the high-energy side. The measured excitation energies and transition probabilities can be adequately described by a single-ion Hamiltonian which includes spin-orbit coupling, trigonal distortion, and an Ising molecular field, taken in the lowest-lying cubic-crystal-field state of ${\mathrm{Fe}}^{2+}$.