THE REACTION BETWEEN H-NB2O5 (C) AND WO3 (G)

The reaction between crystalline H-Nb2O5 and WO3 vapor was investigated at temperatures between 1200 and 1573 K by means of transmission and scanning electron microscopy, optical microscopy, powder X-ray diffraction, and gravimetric techniques. Below 1254 K the reaction taking place was H-Nb2O5 → W3Nb14O44 → W9Nb8O47, while at temperatures above 1254 K the reaction scheme included the phase 6Nb2O5 · WO3, thus: H-Nb2O5 → WNb12O33 → W3Nb14O44 → W9Nb8O47. No other phases were formed. The initial reaction of H-Nb2O5 is homogeneous and takes place throughout the crystal by way of Wadsley-Andersson diffusion steps to give a product phase mostly consisting of disordered rows of (5 × 3)1 and (4 × 3)1 blocks running more or less parallel to the a-axis of H-Nb2O5. The transformation of this material to WNb12O33 and hence to W3Nb14O44 involves similar Wadsley-Andersson shifts which take place in a cooperative fashion to convert one structure to another in a strip by strip fashion. The kinetics of these reactions was recorded. The overall reaction rate followed a simple parabolic rate law despite the complexity of the crystallographic changes taking place. © 1993 Academic Press, Inc.