Cation insertion reactions allow the topochemical reduction of extended transition metal oxide phases. A key requirement of this type of reaction is that the host lattice contains an oxidizable cation and a suitable location to accommodate the inserted cation. This second requirement is particularly onerous (the majority of metal-oxide lattices are dense so cannot accommodate additional cations) and limits the widespread applicability of this approach.

Lithium insertion into cation-deficient perovskite phases

Phases which adopt cation-deficient perovskite structures are rare. However when the B-cation is an early transition metals in its group oxidation state (Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺) phases with A_nB_n-1O_3n cation-deficient perovskite structures can be formed, particularly when the A-cation is barium. The B-cation ‘deficiency’ in these phases is accommodated structurally by sheets of vacant octahedral coordination sites (marked in grey in the figure) which can accommodate intercalated cations.

Reaction of Ba₅Nb₄O₁₅ with LiH at 400 ºC leads to the rapid intercalation of lithium and the formation of Ba₅LiNb₄O₁₅ and the concomitant reduction of niobium. During lithium insertion into Ba₅Nb₄O₁₅ the ccchh stacking sequence of the host lattice is converted into an entirely cubic stacking sequence, while the B-cation vacancy order of the phases is faithfully converted into Li – Nb order in the lithiated product.

The lithiated product exhibits semiconducting behaviour with a temperature dependence consistent with the variable-range-hopping of a small polaron conductor.

Dalton Transactions, 44 (2015) 10636.