Thermally induced structural changes in incommensurate calcium barium niobate Ca0.28Ba0.72Nb2O6 (CBN28)

• Published in: Journal of solid state chemistry 2012-12, Vol.196, p.255-266
• Main Authors: Graetsch, H.A., Schreuer, J., Burianek, M., Mühlberg, M.
• Format: Article
• Language: English
• Subjects: Cations; Condensed matter: structure, mechanical and thermal properties; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; Distortion; Exact sciences and technology; Ferroelectric materials; Ferroelectricity; Incommensaturate crystals; Incommensurate modulation; Inorganic compounds; Materials science; Modulation; Nanostructure; Niobium; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Polar nanodomains; Polyhedrons; Relaxor ferroelectric; Semi-periodic solids; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Tetragonal tungsten bronze
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2012.06.028

The incommensurately modulated crystal structure of relaxor ferroelectric CBN28 was refined at elevated temperatures up to 330°C in the 3+2 dimensional superspace group P4bm(αα1/2,−αα1/2). The structural modulations mainly consisting of cooperative tilting of NbO6 octahedra and an occupational modulation of the large cation site Me2 persist beyond the diffuse ferroelectric transition with slightly reduced amplitudes. A change of symmetry was not observed. Both symmetrically non-equivalent NbO6 octahedra are distorted by off center shifts of the Nb atoms in the same direction along the tetragonal c-axis. The displacements of the Nb atoms are gradually reduced with increasing temperatures until one of the Nb atoms crosses the center of its coordination polyhedron near the transition temperature TM adopting an uncompensated anti-ferroelectric configuration. This change is accompanied by enhanced thermal motions of the Nb atoms along the c-axis. Structural distortions and electric polarization do not completely vanish at TM but may persist in fluctuating polar nanodomains. Cross-over of the off-center shifts of both symmetrically non-equivalent niobium atoms from parallel to anti-parallel configuration in the incommensurately modulated crystal structure of relaxor ferroelectric Ca0.28Ba0.72Nb2O6 (CBN28) at elevated temperatures (left side). The arrows point to columns of octahedra containing Nb1. The remaining octahedra are occupied by Nb2 atoms. [Display omitted] ► The incommensurate structure of relaxor ferroelectric CBN28 was refined at high temperatures. ► The modulations consist of a wave-like occupation of cation sites and displacements of the O atoms. ► The modulations survive the ferroelectric phase transition and no change of symmetry was found. ► Atomic shifts of the ions and the corresponding electric polarization do not completely vanish. ► Thermal motions lead to a cross-over from parallel to anti-parallel configuration of Nb atoms.