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

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.