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, NbO2·500 to NbO2·42, 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.