High-resolution microscopy of nonstoichiometric Nb₂₂O₅₄ crystals: point defects and structural defects

With the use of high-resolution electron microscopy, defects have been observed in crystals of Nb₂₂O₅₄: two types of point defects and extended Wadsley-type defects having a structure corresponding to Nb₁₂O₂₉. 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.