Energy Transfer Processes in “Pure” Mn2+ Fluoride Systems
At very low temperatures (below the Néel point TN), the optical spectra of manganese fluoride systems such as MnF2 and AMnF3 (A = K, Rb, Cs) exhibit fine structures which are related to electronic, magnetic and lattice collective excitations, namely, exciton emissions and their magnon/phonon sidebands. These transitions are intrinsic if they occur at unperturbed magnanese sites. However, small amounts of impurities, typically a few parts per million of Mg, Zn, and Ca, are inevitably present in the crystals. These impurities may perturb the nearest neighbor Mn ions, inducing in the lattice impurity perturbed manganese sites which are populated via the migration of the intrinsic exciton and phonon assisted energy transfer or trapping. The trapped excitons then release their energy giving rise to a number of extrinsic exciton lines and their magnon/phonon sidebands which are observed in the fluorescence spectra.