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

Abstract

In a gas reaction cell (GRC), installed in a high-resolution transmission electron 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 electron beam. Solid state reactions of non-stoichiometric niobium oxides and niobium tungsten oxides with different gases (O 2, H 2 and He) have been carried 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 Nb 12O 29 crystallize in block structures comprising [3×4] blocks. The oxidation of the monoclinic phase occurs via a three step mechanism: firstly, a lamellar defect of composition Nb 11O 27 is formed. Empty rectangular channels in this defect provide the diffusion paths in the subsequent oxidation. In the second step, microdomains of the Nb 22O 54 phase are generated as an intermediate state of the oxidation process. The structure of the final product Nb 10O 25, which consists of [3×3] blocks and tetrahedral coordinated sites, is isostructural to PNb 9O 25. Microdomains of this apparently metastable phase appear as a product of the Nb 22O 54 oxidation. The oxidation reaction of Nb 12O 29 was found to be a reversible process: the reduction of the oxidation product with H 2 results in the formation of the starting Nb 12O 29 structure. On the other hand, the block structure of Nb 12O 29 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 which constitute the solid solution series Nb 8− n W 9+ n O 47 with 0≤ n≤4. All of these phases crystallize in the threefold tetragonal tungsten bronze (TTB) superstructure of Nb 8W 9O 47 ( n=0). In the main reaction, these phases decompose in a gas plasma (O 2, H 2 or He, p=20 mbar) into WO 3− x , which evaporates and solidifies again near the irradiated crystallite, and (Nb,W) 24O 64, which crystallizes in a 2 a superstructure of the TTB type observed here for the first time in the system Nb–W–O. Nb 8W 9O 47, Nb 7W 10O 47 and Nb 6W 11O 47 always react in this way, independent of the applied gas. On the other hand, the treatment of Nb 5W 12O 47 ( n=3) and Nb 4W 13O 47 ( n=4) in an oxygen atmosphere often caused a different 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 WO 3.