Direct observation of lattice defects in H-Nb₂O₅ by high resolution electron microscopy

Detailed two-dimensional lattice images, showing metal-atom positions in crystals of H-Nb₂O₅ quenched from the melt have been observed using a sophisticated electron microscope in both ordered and heavily disordered regions. The blocks in the structure were well ordered along the b axis, but heavily disordered in the ac plane; the sub-cell structure of ReO₃ was still perfectly retained. Four kinds of planar defects, oriented parallel to low-index planes in the host structure, were revealed in the ordered region. Some of these planar defects produce non-stoichiometry. A new polymorph of Nb₂O₅, having the same structure as PNb₉O₂₅, was found in one of the defect structures, and several other new phases of Nb₂O₅ were observed in the heavily disordered region. Incoherently packed blocks and unusual-size blocks played an important role as one of possible origins of the defect planes.