MCD spectroscopy of transition metal ions in fluoride crystals

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Abstract
Absorption spectra of crystals of KZn x Fe1-x F3 (0·006 < 1 - x < 0·07) have been measured in order to obtain information about Fe2+-F--Fe2+ pairs. However, by means of high resolution MCD spectroscopy, it has been established that very small amounts of Fe3+ are present in the crystals. The absorption spectra then show features which are due to (a) Fe2+, (b) Fe3+ and (c) Fe2+, Fe3+ pairs. In addition to the characteristic Jahn-Teller split 5 T 2g → 5 Eg band of Fe2+, there is a weak, spin-allowed, vibronically assisted transition near 40 000 cm-1, which is assigned to 5 T 2g (d6) → 5 A 1g (d5s). The MCD of this band is consistent with this assignment. The remainder of the visible and near ultra-violet absorption intensity is associated with Fe2+/Fe3+ pairs. Most of the bands are due to excitations localized on the Fe2+ though some weaker ones are assigned as excitations to 4 A 1g on the Fe3+ of the pair. There is also a strong, broad underlying absorption probably due to t 2g → t 2g charge transfer in the pair.