In addition to being inserted or extracted, the oxide anions in extended solids can also be exchanged for different anions via topochemical reactions. By performing aliovalent anion exchange reactions (replacing O^2- oxide ions with anions of a different charge) both the oxidation states and local coordination environments of metal centers within extended solids can be modified, thus modifying the physical behaviour of these materials.

Oxide-Hydride Formation

In addition to topochemical reductions, binary metal hydrides can bring about anion-exchange reactions to yield oxide-hydride phases. For example reaction between LaSrCoO₄ and CaH₂ initial forms the reduced phase LaSrCoO_3.35. Further heating facilitates an anion-exchange reaction to yield LaSrCoO₃H_0.7, which was the first transition metal oxide-hydride phase to be reported.

Chemistry of Materials, 12 (2000) 2182.
Science, 295 (2002) 1882.

More recently we have prepared SrVO₂H by reaction between SrVO₃ and CaH₂. SrVO₂H is the first stoichiometric, anion-ordered, transition metal oxide-hydride phase reported. The oxide-hydride anion order in this phase means the structure of SrVO₂H consists of V³⁺ cations located within square-planes of oxide ions. These V³⁺O₄ units share corners to form infinite VO₂ sheets directly analogous the CuO₂ planes observed in Sr_1-xCa_xCuO₂, the parent phase of the high-T_c superconducting cuprates. However unlike Sr_1-xCa_xCuO₂ the VO₂ sheets in SrVO₂H are connected via the hydride ions which occupy the remaining two coordination sites around each V³⁺ center.

Despite the formal 6-fold VO₄H₂ coordination of the vanadium centers in SrVO₂H, there is a direct structural and electronic analogy between this phase and the infinite-layer ABO₂ phases. This is because the 1s valence orbitals of the hydride ions have strict sigma-type symmetry with respect to the vanadium cations and are thus orthogonal to the pi symmetry d_xz, d_yz and d_xy orbitals from which the HOMO (a degenerate (d_xz, d_yz)² pair) and LUMO (the d_xy orbital) of the local VO₄H₂ unit are derived. Thus to a first approximation the d-electrons in SrVO₂H only ‘see’ the infinite layer V-O framework, as the sigma-type vanadium d-orbitals which do interact with the H 1s orbitals are empty and energetically remote from the filled orbitals. As a result SrVO₂H can be considered directly analogous to a d² infinite-layer system, such as the hypothetical phase ‘KVO₂’ in which the SrH layers are replaced by layers of potassium cations.

Angewandte Chemie, 53 (2014) 7556.

2-Step Anion Excahnge

Anions in complex oxide phases can also be substituted by a 2-step reduce-then-oxidize route. For example, Sr₇Mn₄O₁₅ can be topochemically reduced to the anion-vacancy ordered phase Sr₇Mn₄O₁₃. Subsequent fluorination of this reduced material yields the anion-ordered oxide-fluoride Sr₇Mn₄O₁₃F₂ in which the oxide/fluoride ordering arrangement is patterned by the anion-vacancy order in the reduced intermediate phase.

Journal of Solid State Chemistry, 180 (2007) 2851.
Inorganic Chemistry, 47 (2008) 5212.