The Fe2+ fraction observed in the Mössbauer emission spectra of57Co:LiNbO3 and57Co:LiTaO3 exhibits both slow electronic relaxation and nonthermal populations of thems sublevels of the5Alg orbital singlet ground state at low temperatures (T<15 K) in high magnetic fields. The relaxation rates depend on temperature and on the angle ϕ between the magnetic field and the crystallographicc-axis.

Mössbauer emission spectra of57Co: LiTaO3 show as a consequence of the nuclear decay anomalous emission line intensities in the subspectra of Fe2+ and Fe3+. Magnetic field and angle dependence of Fe3+ intensities can be explained by taking into account polarization-and crystal-field effects in the excited states. The experimental results for LiTaO3 are in good agreement with theory and previous measurements on LiNbO3.

Mssbauer emission spectra of LiNbO3:57Co single crystals at 100 K in a magnetic field of 4 T show Fe3+ line intensities corresponding to a nearly Boltzmann population of the6A1g Zeeman sublevels. Supposing that this is due to a spin-lattice relaxation in the ground state, no relaxation matrix can reproduce the shape of the spectrum. We conclude that the initial populations are temperature dependent due to spin-lattice relaxation within the $$\Gamma _6 ^T $$ excited doublet.

M\"ossbauer emission spectra of LiNb${\mathrm{O}}_{3}$:$^{57}\mathrm{Co}$ single crystals in magnetic fields up to 5 T at 4.2 K show initial populations of the $^{6}A_{1\mathrm{g}}$ Zeeman substates of ${\mathrm{Fe}}^{3+}$ which depend strongly on the angle between the crystallographic $c$ axis and the magnetic field. This is interpreted in terms of a crystal-field effect on excited states which influences the initial populations of the Zeeman sublevels of the $^{6}A_{1\mathrm{g}}$ ground term after the electron-capture decay of $^{57}\mathrm{Co}$. An intersystem crossing process due to orbit-lattice interaction can fairly well explain the angular dependence.

The magnetic field dependence of the anomalous Fe3+ emission line intensities in LiNbO3:57Co cannot be due to a direct spin-lattice relaxation process in the6S ground state. Raman and Orbach processes in the ground state are ruled out by the temperature independent behaviour of the spectra. The observed line intensities are proportional to the initial populations of the corresponding Zeeman levels.