Reduction and non-stoicheiometry of niobium pentoxide

Abstract

The reduction equilibria and non-stoicheiometry of niobium pentoxide have been re-examined in order to correlate thermodynamic behaviour with structural modification during reaction. Equilibria were measured thermogravimetrically, by (T,x) scans in constant gas buffer mixtures, over oxygen activities 10^–15–10^–19 atm at 900–1050 °C. Products of microbalance runs and of parallel preparations were characterized by electron diffraction and direct lattice imaging electron microscopy. The composition range traversed, NbO_2·500 to NbO_2·42, simulated the reversible bivariant behaviour of a non-stoicheiometric oxide, but structurally it was converted progressively from the H form of Nb₂O₅ to Nb₂₅O₆₂ to Nb₁₂O₂₉. These structures were generated in coherently interrgrown lamellae and domains; the crystals do not behave as classical Gibbs phases, and the thermodynamic behaviour arises, in part, from gross hysteresis and ‘hybrid crystal’ formation. The structures formed at 950–1050 °C are not in inner equilibrium, and at 1200–1300 °C rearrange to coherent intergrowth phases, Nb₅₃O₁₃₂ and Nb₃₉O₉₇. It is inferred that two distinct reaction processes are involved: an easy process that transforms one simple block structure into another, and a slow reshuffle of crystallographic shear planes to attain inner equilibrium. Some stoicheiometric variability of the structures is not excluded.