IN-SITU CHEMICAL-REACTION OF NB12O29 IN A HIGH-RESOLUTION GAS REACTION CELL MICROSCOPE

Solid-state oxidation-reduction of a nonstoichiometric Nb12O29 (NbO2.4 17) with monoclinic cell was studied using a JEOL 4000 EX HRTEM equipp ed with a gas reaction cell in which the oxide was oxidized and then r educed by sequentially introducing O-2 and H-2. The oxidation yielded other block structures with higher oxygen content, The first step of t he oxidation mechanism, involving formation of lamellar defects of com position Nb11O27 (NbO2.450), could be elucidated with great certainty, Nb10O25 (a metastable polymorph of NbO2.500) was formed as a final st age of the oxidation process, Subsequent reduction led to a nearly ord ered Nb12O29 phase suggesting that such an oxidation-reduction reactio n is a reversible process, When Nb12O29 was directly reduced, NbO was found to grow on the surface of the crystal. (C) 1996 Academic Press, Inc.