Temperature dependence of incommensurate modulation in Ca0.28Ba0.72Nb2O6.

We present an electron microscopy and diffraction study of a Ca x Ba 1 − x Nb 2 O 6 ceramic with x = 0.28 (CBN28), a ferroelectric material with a partially filled tetragonal tungsten-bronze structure. The microstructure has strong similarities to that of Sr x Ba 1 − x Nb 2 O 6 , with an average orthorhombic symmetry and an intimate intermixture of merohedral twin variants at a length scale of tens of nanometers. Superstructure spots in diffraction patterns are displaced by a one-dimensional incommensurate modulation, characterized by a propagation vector δ δ 0. Heating experiments show that δ is strongly coupled to ferroelectric polarization, decreasing as 180 ° ferroelectric domains become more finely spaced and needle-like as the Curie temperature T C is approached during heating and increasing once more above T C. No change in symmetry is observed at T C , consistent with a transition from ferroelectric to antiferroelectric (or ferrielectric) relaxor properties. The superstructure spots and incommensurate modulation disappear ∼ 250 ° C above T C , consistent with polar regions in the material becoming fully transient. [ABSTRACT FROM AUTHOR]