Freudenbergite - a new example of electron hopping

We have recently found that samples of freudenbergite with a mixed ferric-ferrous composition exhibit considerable electron hopping at room temperature as evidenced in ⁵⁷FeMössbauer spectra. Freudenbergite is most commonly ferric with composition Na₂Fe³⁺₂Ti₆O₁₆, but can be ferrous, Na₂Fe²⁺Ti₇O₁₆, with a complete solid solution possible between these end members. Since both iron and titanium can have two valences, it is of importance to understand whether the electron hopping is between neighbouring Fe²⁺ and Fe³⁺ ions or whether it involves titanium in the form Fe²⁺ + Ti⁴⁺ ↔ Fe³⁺ + Ti³⁺. To try and distinguish between these possibilities, we will discuss the differences in the optical reflectance spectra between the black, electron hopping samples and the grey-green static samples. In addition, we will show new Mössbauer spectra for different freudenbergite compositions. Application of a magnetic field of 1.6 T did not change the proportion of the electron hopping contribution. The measured hyperfine field was enhanced above the applied field for both sites. Without the applied field, the ferrous contribution had to be fitted to two doublets, possibly representing the M1 and M2 sites, but with the applied field, they were able to be fitted as a single contribution. For the ferric sites, the M1 and M2 contributions were indistinguishable in both cases. On strongly heating a mixed valence sample, nearly half the iron was expelled as hematite, while the remainder was still contained as a titanate. (author)