CRYSTAL-STRUCTURE ANALYSIS AND POLARIZATION MECHANISMS OF FERROELECTRIC TETRAGONAL TUNGSTEN BRONZE LEAD BARIUM NIOBATE

Lead barium niobate single crystals of the composition Pb0.596Ba0.404N b2.037O6, near the tetragonal: orthorhombic morphotropic phase boundar y with tetragonal symmetry at room temperature, were the subject of th is study. The crystallographic details, including the ionic coordinati on, the site occupancies and the thermal anisotropy of a lead-containi ng tetragonal tungsten bronze crystal are determined. This paper repor ts the structural refinement results, the site-preference by cations a nd vacancies, the macroscopic polarization properties derived from the cation displacement, and the apparent valences determined by the bond ing distance. Unlike other non-lead-containing ferroelectric tungsten bronzes, significant polarization contribution can be attributed to ca tions (Pb2+ and Ba2+) at the A2-site. Large anisotropic thermal coeffi cients found for cations at the A2-site further strengthens the local polarization fluctuation model and suggests the local polarization com ponents are perpendicular to the mirror plane along the [110] directio n.