Nb Solution within Bi4Ti3O12 Sub-structure in the Intergrowth Bismuth-layered Compound Bi7Ti4NbO21

In intergrowth bismuth-layered compound Bi7Ti4NbO21, growth defects, such as disordered intergrowth with extra layers of Bi4Ti3O12 or Bi3NbTiO9 constituent sub-structures, or as the co-growth of Bi7Ti4NbO21 onto Bi4Ti3O12 grains, were frequently observed using high-resolution transmission electron microscope (HRTEM)[1]. In order to further find evidence to support the re-ordering picture that was proposed to explain formation of the intergrowth and associated defects, we employ the low- and medium-resolution high-angle annular-dark-field (HAADF) imaging combined with the quantitative energy dispersive X-ray spectroscope (EDXS) analysis to probe into a heavily defected intergrowth structure. A gradual transformation from the ordered intergrowth of both sub-structures to the dominance of Bi4Ti3O12 sub-structure was observed. A substantial level of Nb solution could be detected in n-layered (n≥2) Bi4Ti3O12 sub-structure by spatially-resolved compositional quantification to differentiate the contribution from the adjacent single Bi3NbTiO9 layer. The presence of a finite Nb concentration in the Bi4Ti3O12 sub-structure indicates a substantial and uniform cations inter-change occurred between the two sub-structures, which is inherited most likely from the parent phases via the partially dissolved sintering melts.