The Mössbauer effect observed with LiNbO3:Co57 (source) and LiNbO3:Fe57 (absorber) crystals showed the existence of high-spin Fe2+ and Fe3+ valence states. The Fe2+/Fe3+ ratio could be changed by reducing or oxidizing heat treatment. Fe3+ in sources and absorbers shows Mössbauer spectra which are typical for slow electronic relaxation between the crystal field states of the 6S5/2 state ion. For both Fe3+ and Fe2+, the principal axis of the electric field gradient is found to be parallel to the crystallographic c axis. For ferric iron Vzz is positive, while Vzz is negative and strongly temperature dependent for ferrous iron. The impurity site substitution and charge compensation mechanism are discussed.
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Keune et al. (1975) studied this question.
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