The structure of the bronze Na 13Nb 35O 94 and the geometry of ferroelectric domains

Abstract

The composition and structure of the oxide Na 13Nb 35O 94 has been determined from single crystal X-ray diffraction data. The unit cell is based on three tetragonal-bronze subcells in which all four- and five-sided tunnels are filled with interstitial ions. The niobium atoms exhibit square pyramidal, octahedral and pentagonal bipyramidal coordination while sodium ions, in the main, have square or trigonal prismatic coordination. The space group is polar, Pba2, and each niobium atom is displaced from its basal octahedral plane by up to 0.12 Å in the same direction along [001]. The domain structure and orientation of domain boundaries, as revealed by electron transmission micrographs, are explained in terms of a reversal of polarity of adjacent ferroelectric domains.