Hydride Reductions of Transition Metal Oxides

Abstract

Abstract Low-temperature reactions are a powerful approach to generate new transition metal oxides that are inaccessible by conventional high-temperature reactions. In this review, we describe the recent progress of the topochemical reduction method using metal hydrides for transition metal oxides, in particular, focusing on structural modifications (relations), chemical and physical properties, and the factors that direct selective and rational preparations. The hydride reduction has been so far extensively applied to 3d transition metal perovskite oxides, yielding highly reduced products with unusual coordination environment (e.g., FeO4 square-planar coordination), and extremely low-valent metal centers (e.g., Mn(I) and Co(I)). Non-perovskite oxides like pyrochlore and hexagonal perovskite can be also reduced. Moreover, this method allows access to oxyhydride materials (LaSrCoO3H0.7 and BaTiO3−xHx) that are promising for use as hydride ion conductors. Morphology-controlled oxides (thin film- and nano-oxides) are useful targets for hydride reduction, opening new possibilities for extending functions.