Die Modifikationen des Niobpentoxids

Heteroepitaxial Growth of T-Nb₂O₅ on SrTiO₃

There is a growing interest in exploiting the functional properties of niobium oxides in general and of the T-Nb₂O₅ polymorph in particular. Fundamental investigations of the properties of niobium oxides are, however, hindered by the availability of materials with sufficient structural perfection. It is expected that high-quality T-Nb₂O₅ can be made using heteroepitaxial growth. Here, we investigated the epitaxial growth of T-Nb₂O₅ on a prototype perovskite oxide, SrTiO₃. Even though there exists a reasonable lattice mismatch in one crystallographic direction, these materials have a significant difference in crystal structure: SrTiO₃ is cubic, whereas T-Nb₂O₅ is orthorhombic. It is found that this difference in symmetry results in the formation of domains that have the T-Nb₂O₅ c-axis aligned with the SrTiO₃ <001>_s in-plane directions. Hence, the number of domain orientations is four and two for the growth on (100)_s- and (110)_s-oriented substrates, respectively. Interestingly, the out-of-plane growth direction remains the same for both substrate orientations, suggesting a weak interfacial coupling between the two materials. Despite challenges associated with the heteroepitaxial growth of T-Nb₂O₅, the T-Nb₂O₅ films presented in this paper are a significant improvement in terms of structural quality compared to their polycrystalline counterparts.

Dislocations and related defects in niobium oxide structures

Lattice images of the niobium oxides, structures based on the linkage of octahedral groups in continuous networks, occasionally contain features recognizable as dislocations. Since lattice imaging enables the micro-structure to be resolved in great detail, at the level of local structural organization, it is possible to determine the configuration, and also to infer the chemical composition, of dislocated areas. By treating the niobium oxide ‘block’ structures as superstructures of the ReO 3 (DO 9 ) type, the topology of dislocations can be expressed by relations between the insertion (or deletion) of one or more half-planes of cations, or of oxygen atoms only, changes in the number of crystallographic shear plane interfaces between blocks or columns, changes in (idealized) dimensions and any requisite distortion in the third dimension. Mapping the structure around a dislocation, from the lattice image, is directly equivalent to plotting the Burgers’ circuit. In this way, the precise nature of a dislocating perturbation and its implications for the local chemical composition of the crystal can be directly identified. The method is exemplified by analysis of dislocations and of related extended defects of several types, associated with twinning phenomena, semicoherent intergrowth between different ReO 3 -type superstructures and arrays building up a low angle boundary. The essential features of the analysis are not restricted to structures of the niobium oxide type, but can be extended to other types of polyhedron networks.