Solid-gas reactions of complex oxides inside an environmental high-resolution transmission electron microscope

In a gas reaction cell (GRC), installed in a high-resolution transmission e lectron microscope (HRTEM) (JEOL 4000EX), samples can be manipulated in an ambient atmosphere (p < 50 mbar). This experimental setup permits not only the observation of solid-gas reactions in situ at close to the atomic level but also the induction of structural modifications under the influence of a plasma, generated by the ionization of gas particles by an intense electr on beam. Solid state reactions of non-stoichiometric niobium oxides and nio bium tungsten oxides with different gases (O-2, H-2 and He) have been carri ed out inside this controlled environment transmission electron microscope (CETEM), and this has led to reaction products with novel structures which are not accessible by conventional solid state synthesis methods. Monoclinic and orthorhombic Nb12O29 crystallize in block structures compris ing [3 x 4] blocks. The oxidation of the: monoclinic phase occurs via a thr ee step mechanism: firstly, a lamellar defect of composition Nb11O27 is for med. Empty rectangular channels in this defect provide the diffusion paths in the subsequent oxidation. In the second step, microdomains of the Nb22O5 4 phase are generated as an intermediate state of the oxidation process. Th e structure of the final product Nb10O25, which consists of [3 x 3] blocks and tetrahedral coordinated sites, is isostructural to PNb9O25. Microdomain s of this apparently metastable phase appear as a product of the Nb22O54 ox idation. The oxidation reaction of Nb12O29 was found to be a reversible pro cess: the reduction of the oxidation product with H-2 results in the format ion of the starting Nb12O29 structure. On the other hand, the block structu re of Nb12O29 has been destroyed by a direct treatment of the sample with H -2 while NbO in a cubic rock salt structure is produced. This in situ technique has also been applied to niobium tungsten oxides whi ch constitute the solid solution series Nb8-nW9+nO47 With 0 <less than or e qual to> n less than or equal to 4. All of these phases crystallize in the threefold tetragonal tungsten bronze (TTB) superstructure of Nb8W9O47 (n = 0). In the main reaction, these phases decompose in a gas plasma (0(2), H-2 Or He, p = 20 mbar) into WO3-x, which evaporates and solidifies again near the irradiated crystallite, and (Nb,W)(24)O-64, which crystallizes in a ro ot 2a superstructure of the TTB type observed here for the first time in th e system Nb-W-O. Nb8W9O47,Nb7W10O47 and Nb6W11O47 always react in this way, independent of the applied gas. On the other hand, the treatment of Nb5W12 O47 (n = 3) and Nb4W13O47 (n = 4) in an oxygen atmosphere often caused a di fferent reaction: these phases have been oxidized and a heavily disordered bronze-type structure has been formed. The oxygen excess in these products is largely accommodated in segregated domains of WO3. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.