High-resolution microscopy of nonstoichiometric Nb₂₂O₅₄crystals: point defects and structural defects

Abstract

With the use of high-resolution electron microscopy, defects have been observed in crystals of Nb22054" two types of point defects and extended Wadsley-type defects having a structure corresponding to Nb12029. For one type of point defect a model is proposed which contains two displaced metal atoms and two interstitial oxygen atoms within a 'block'. For the other, one metal atom and three oxygen atoms are considered to be missing from a tetrahedral site. Contributions made by the three types of defects to the non-stoichiometry of the crystals are discussed.

Refinement of the defect structure of `GeNb9O25' by high-resolution electron microscopy

“…The results of the analysis of the atom configurations in 'GeNbgO2s' have significance for the understanding of other structures. The images of crystals of Nb22054 which have a slight excess of metal atoms show considerable fluctuations in the contrast at the tetrahedral positions (Iijima, Kimura & Goto, 1974). Similar results have been observed in images of crystals of Nb25062 which also have a slight excess of metal (Fig.…”

“…Allpress & Roth (1971) reported that intimate intergrowths of structure slightly different from that of the host crystal (Wadsley defects) may play an important role. Later one of the present authors (Iijima, Kimura & Goto, 1974) demonstrated experimentally that Wadsley defects may not be adequate to account for the nonstoichiometry in some cases but point defects may also exist. In particular, considerable fluctuations of contrast were observed at points corresponding to the presumed sites of tetrahedrally coordinated metal atoms in Nb22+xO54 crystals and it was concluded that these could indicate the presence of point defects in the related columns of atoms.…”

“…We therefore suspect that this disorder in the tetrahedral columns may be associated with their nonstoichiometry. In fact the presence of such defects in addition to Wadsley defects has been indicated experimentally (Allpress & Roth, 1971 ;Iijima, Kimura & Goto, 1974).…”

The ultrastructure of pyroxenoid chain silicates. II. Direct structure imaging of the minerals rhodonite and wollastonite

“…Direct interpretation of defects in oxides with corner-sharing octahedra is then possible (e.g. Iijima & Allpress, 1974;Iijima, Kimura & Goto, 1974) but not where edge sharing of octahedra occurs, as in pyroxenoids.…”