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

Abstract

We present an electron microscopy and diffraction study of a CaxBa1−xNb2O6 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 SrxBa1−xNb2O6, 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 δδ0. 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 TC is approached during heating and increasing once more above TC. No change in symmetry is observed at TC, consistent with a transition from ferroelectric to antiferroelectric (or ferrielectric) relaxor properties. The superstructure spots and incommensurate modulation disappear ∼250°C above TC, consistent with polar regions in the material becoming fully transient.