Anion insertion reactions allow the topochemical oxidation of extended transition metal oxide phases. A key requirement of this type of reaction is that the host lattice contains an oxidizable cation an a suitable location to accommodate the inserted anions. Given the similar ionic radii of O^2- and F^- both fluoride and oxide ions can be readily inserted into complex oxide lattices.

Anion insertion into cation-ordered perovskite phases

Cation-ordered anion-vacancy ordered perovskite phases (see are work in this area here) naturally contain sites suitable for the accommodation of oxidatively intercalated anions, and can show remarkable selectivity in the reactivity towards inserted anions.

For example the two isostructural phases Ba₂YFeO₅ and Ba₂YCoO₅ exhibit strikingly different reactivity to oxygen and fluorine – Ba₂YFeO₅ intercalates oxygen and not fluorine, while Ba₂YCoO₅ intercalates fluorine and not oxygen.

The oxidation of Ba₂YFeO₅ breaks the inversion symmetry of the host lattice and as a result Ba₂YFeO_5.5 exhibits both pyroelectric and ferromagnetic behaviour, and demonstrates that topochemical oxidation reactions offer a novel route to the preparation of multiferroic materials.

Inorganic Chemistry, 51 (2012) 12281.
Chemistry of Materials, 25 (2013) 1800.
Inorganic Chemistry, 52 (2013) 13762.

Anion insertion into Ruddlesden-Popper structured oxides

Extended oxide phases with A_n+1B_nO_3n+1 Ruddlesden-Popper type structures contain ‘empty’ anion intercalation sites within the ‘rock salt’ layers of their frameworks. Under the correct conditions, additional anions can be inserted into these coordination sites leading to a net oxidation of the phase. An interesting feature of this type of anion insertion is that the inserted anions are not located within the primary coordination sphere of the metal cations which are oxidized. This process can therefore be considered to be an ‘electronic only’ type oxidation.

By applying this approach to Sr₃MRuO₇ (M = Ti, Mn, Fe) Ruddlesden-Popper phases we have prepared the analogous oxidized materials. In case of the manganese and titanium phases a simple oxidation reaction occurs to form Sr₃MRuO₇F₂ (M = Ti, Mn) in which fluorine is inserted into materials in a simple topochemical oxidation reaction to form Ru⁶⁺ phases. In contrast fluorination of Sr₃FeRuO₇ results in Sr₃FeRuO_5.5F_3.5 a phase which contains Fe³⁺ and Ru^5.5+ and in which in addition to anion insertion, some oxide-for-fluoride anion exchange has also occurred.

Inorganic Chemistry, 52 (2013) 3388.