Thermally induced structural changes in incommensurate calcium barium niobate Ca{sub 0.28}Ba{sub 0.72}Nb{sub 2}O{sub 6} (CBN28)
- Institut fuer Geologie, Mineralogie und Geophysik der Ruhr-Universitaet Bochum (Germany)
- Institut fuer Kristallographie der Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln Germany (Germany)
The incommensurately modulated crystal structure of relaxor ferroelectric CBN28 was refined at elevated temperatures up to 330 Degree-Sign C in the 3+2 dimensional superspace group P4bm({alpha}{alpha}1/2,-{alpha}{alpha}1/2). The structural modulations mainly consisting of cooperative tilting of NbO{sub 6} 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 NbO{sub 6} 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 T{sub M} 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 T{sub M} but may persist in fluctuating polar nanodomains. - Graphical abstract: 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 Ca{sub 0.28}Ba{sub 0.72}Nb{sub 2}O{sub 6} (CBN28) at elevated temperatures (left side). The arrows point to columns of octahedra containing Nb1. The remaining octahedra are occupied by Nb2 atoms. Highlights: Black-Right-Pointing-Pointer The incommensurate structure of relaxor ferroelectric CBN28 was refined at high temperatures. Black-Right-Pointing-Pointer The modulations consist of a wave-like occupation of cation sites and displacements of the O atoms. Black-Right-Pointing-Pointer The modulations survive the ferroelectric phase transition and no change of symmetry was found. Black-Right-Pointing-Pointer Atomic shifts of the ions and the corresponding electric polarization do not completely vanish. Black-Right-Pointing-Pointer Thermal motions lead to a cross-over from parallel to anti-parallel configuration of Nb atoms.
- OSTI ID:
- 22149940
- Journal Information:
- Journal of Solid State Chemistry, Vol. 196; Other Information: Copyright (c) 2012 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
Similar Records
Incommensurate structure of GdBaCo{sub 2}O{sub 5+{delta}}({delta}{approx}0.38)
A quaternary lead based perovskite structured materials with diffuse phase transition behavior