NONEQUILIBRIUM REACTIONS OF ALPHA-PHASE PROX WITH O-2

In this paper we use electron diffraction and high-resolution imaging techniques to study the structural consequences of rapid cooling, to room temperature, of fine particles of praseodymium oxide prepared as the disordered alpha-phase in the composition range from PrO1.78 to PrO1.83. These experiments reveal that particles of the disordered fluorite phase, prepared at higher temperatures, pass through many steps of order as they cool and oxidize to ordered intermediate structures in their near-surface regions. The entities that bring about this transformation are condensed vacant oxygen sites that segregate on {220} planes and finally order to give members of the even-n homologous series PrnO2n-2. The unit cells for new members of the series with n assumed to be 16 are reported.