Abstract
The optical absorption spectra of single crystals of MgCl2 containing Fe2+ and Co2+ have been measured and the temperature dependence of the bands has been studied. For the transitions giving rise to hot bands, the energy of the initial state has been deduced from the temperature dependence of the integrated absorption. For MgCl2:Co2+, spectra in the region of 19000 cm-1 are presented, and one hot band has been identified. The spectrum of MgCl2:Fe2+ shows several hot bands in the region of 23600 cm-1. The energies found for the initial states of all these transitions agree well with the ground-state splittings calculated using the nearest-neighbour exchange parameters appropriate to pure FeCl2 and CoCl2. The spectrum of MgCl2:Fe2++Co2+ has an additional hot band at 23558 cm-1 which has a lower state at 3.6 cm-1. A simple model of the Fe2+-Co2+ exchange using the geometric mean approximation predicts an energy for this state of 3.8 cm-1, in good agreement with the experimental result.
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