Crystal structure analysis and polarization mechanisms of ferroelectric tetragonal tungsten bronze lead barium niobate

Lead barium niobate single crystals of the composition Pb0.596Ba0.404Nb2.037O6, near the tetragonal: orthorhombic morphotropic phase boundary with tetragonal symmetry at room temperature, were the subject of this study. The crystallographic details, including the ionic coordination, the site occupancies and the thermal anisotropy of a lead-containing tetragonal tungsten bronze crystal are determined. This paper reports the structural refinement results, the site-preference by cations and vacancies, the macroscopic polarization properties derived from the cation displacement, and the apparent valences determined by the bonding distance. Unlike other non-lead-containing ferroelectric tungsten bronzes, significant polarization contribution can be attributed to cations (Pb2+ and Ba2+) at the A2-site. Large anisotropic thermal coefficients found for cations at the A2-site further strengthens the local polarization fluctuation model and suggests the local polarization components are perpendicular to the mirror plane along the [110] direction.