Raman spectroscopy studies of antiferromagnetic FeCO3 and related carbonates

Raman spectroscopy experiments were performed on antiferromagnetic siderite (natural FeCO₃). Weak lines at room temperature (in addition to the expected vibrational lines) were found to be seven well defined excitations at liquid helium temperature. Polarization tensor components of these new lines were examined at temperatures varying between room and liquid helium temperature. Frequency decreases upon cooling were observed for three of the lines (the greatest change occuring near the Néel temperature, 38 K). By comparison with infrared spectra, variable temperature Raman spectra and impurity analysis of two related crystals (antiferromagnetic MnCO₃ and CaMg(CO₃)₂ containing 6% iron), new explanations for two (741 and 1735 cm^?1) of three previously observed lines and for one (870cm^?1) of the remaining four are presented. The three variable frequency lines (440,1175 and 1225 cm^?1) are considered magnetic excitations between trigonal field, spin-orbit, and exchange split states of the ferrous ion. The frequency decreases upon cooling may be due to unquenched orbital angular momentum resulting in an exchange interaction of a non-Heisenberg form. Symmetry distortion due to magnetic ordering upon cooling may cause the infrared 741 cm^?1 vibration to become Raman-active.