Low-Temperature Topochemical Reactions as a Route to Novel Mixed-Anion Materials

The vast majority of complex solids are prepared at high temperatures under reaction conditions that apply ‘thermodynamic control’ to the selection of reaction products. By utilizing the ‘extreme chemical conditions’ of low-temperature, and very low <i>p</i>O₂ it is possible to prepare a range of novel solids which are metastable and cannot be synthesised under more conventional conditions.<br/>By using these synthetic approaches it is possible prepare extended solids containing transition metal cations with unusual combinations of oxidation state and coordination geometry (e.g. square-planar Ni¹⁺, Fe²⁺, Ru²⁺, Ir²⁺). In addition, these conditions can also facilitate the substitution of oxide anions by hydride anions, to form transition-metal oxyhydride phases. The differing charges of O^2- and H^- mean that any hydride-for-oxide anion exchange is necessarily reductive. Furthermore, the greater polarizability in combination with the lower electronegativity of hydride anions means that M-H bonds have a greater degree of covalency and orbital mixing than the corresponding M-O bonds, and as a result the band structures of oxide-hydride phases are qualitatively different to all-oxide systems, and the strength of magnetic exchange couplings is also enhanced.<br/>A further, more subtle, difference between oxide and hydride is the lack of π-symmetry valence orbitals in the later anion. This means that hydride ions cannot be involved in bond formation with metal orbitals of π-symmetry. This can have a dramatic influence on the orbital connectivity of oxide-hydride phases, particularly when there is extensive anion order, as can be seen in the highly 2-dimensional character of the transport in the high-pressure metallic state of SrVO₂H.<br/>The structures and physical properties of a range of novel 3d and 3d/4d and 3d/5d reduced transition-metal oxides and oxyhydrides will be described.