MCD spectroscopy of transition metal ions in fluoride crystals

Abstract

Absorption spectra of crystals of KZn _x Fe_1-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 E_g 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.