Transmission Electron Microscopy Evidence of Spontaneous B-Cation Layered Distribution in NaNb1−xTaxO3

摘要

A transmission electron microscopy (TEM) study of the complex NaNb_1−xTa_xO₃ (0.4 ≤ x ≤ 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb_1−xTa_xO₃ oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d0 octahedrally coordinated cations, they show a propensity to second-order Jahn−Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium−tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice.