The topochemical deintercalation of oxide ions from extended transition metal oxide lattices changes both the oxidation state of transition metal centres, and their local coordination environments – two features which have a decisive influence on the electronic and magnetic behaviour of phases. We have demonstrated that binary metal hydrides (NaH, LiH, CaH₂) can be used to deintercalate oxide ions from complex transition metal oxide phases to yield highly metastable phases which contain transition-metal centres in highly unusual oxidation states and/or coordination environments.

Unusual Transition-Metal Oxidation States

We have demonstrated that binary metal hydrides (NaH, LiH, CaH₂) can be used to deintercalate oxide ions from complex transition metal oxide phases to yield highly metastable phases which contain transition-metal centres in highly unusual oxidation states.

For example, by reacting the Mn^3+/4+ perovskite phase La_1-xCa_xMnO₃ (0.6 < x < 1) with NaH at 210 ºC, oxygen can deintercalated from the structure to form La_1-xCa_xMnO₂ - a material which retains the cation lattice of the ‘parent’ phase but has a lower oxygen stoichiometry. During the reaction the oxide ion lattice reorganises itself to retain the octahedral coordination of the manganese cations to yield a material which can be thought of as a cation-ordered variant of the rock-salt structure. The decrease in the oxygen stoichiometry reduces the manganese cations, so they have an average oxidation state less than Mn²⁺. This is the first reported example of Mn¹⁺ in an extended oxide phase.

In contrast the reaction of SrFe_0.5Ru_0.5O₃ with CaH₂ at 400 ºC yields SrFe_0.5Ru_0.5O₂, a phase in which both iron and ruthenium occupy square-planar coordination sites and have both been reduced to the divalent oxidation state. This is the first example of Ru²⁺ in an extended oxide. Magnetisation measurements and DFT calculations indicate the d⁶ Fe²⁺ centres adopt a high-spin S = 2 configuration, while in contrast the d⁶ Ru²⁺ centres adopt an intermediate spin S = 1 configuration, leading to net spin-glass behaviour.

Journal of the American Chemical Society 133 (2011) 18397.
Journal of the American Chemical Society 135 (2013) 1838.

Unusual Transition-Metal Coordination Geometries

The local coordination geometry of ligands around transition metal cations defines the relative energies of the metal d-orbitals, and thus has a strong influence on the electronic structure and behaviour of extended metal oxides.

The topochemical nature of low-temperature anion deintercalation reactions means that phases prepared in this way are not restricted to forming only the most energetically favourable coordination polyhedra (e.g. octahedra and tetrahedra) but can instead form materials with more exotic point symmetries at metal centres.

For example, reaction of the oxy-chloride phase Sr₃Fe₂O₅Cl₂ with LiH yields Sr₃Fe₂O₄Cl₂, a material containing sheets of apex-sharing FeO₄ square planes. Further investigation reveals that the local coordination of the iron centres in Sr₃Fe₂O₄Cl₂ is directed by the surrounding lattice, so that by replacing the iron with cobalt, the isostructural phase Sr₃Co₂O₄Cl₂, containing square-planar Co²⁺, can be prepared. The square-planar Co²⁺ centres adopt an orbitally degenerate electronic configuration so that the system undergoes a structural symmetry breaking transition below 200K, due to a cooperative Jahn-Teller distortion.

The influence of ‘spectator’ cations can also be seen during the reduction of REBaCo₂O₅ phases. Reaction of YBaCo₂O₅ with NaH yields YBaCo₂O_4.5, a Co²⁺ phase which contains CoO₄ tetrahedra and CoO₅ square-based pyramids. In contrast the reduction of the isostructural phase LaBaCo₂O₅ yields LaBaCo₂O_4.25 a phase that not only contains Co²⁺ centres in CoO₄ tetrahedra and CoO₅ square-based pyramids, but also Co¹⁺ centres in square-planar coordination sites.This demonstrates that the identity of spectator cations can influence both the local metal coordination geometries and oxygen stoichiometry of product phases.

Inorganic Chemistry 49 (2010) 11062.
Journal of the American Chemical Society 134 (2012) 15946.
Journal of the American Chemical Society 132 (2010) 2802.