Crystal structure analysis of ferroelectric tetragonal tungsten bronze Pb0.596Ba0.404Nb2.037O6

Lead barium niobate single crystals of the composition Pb_0.596Ba_0.404Nb_2.037O₆ near the tetragonal orthorhombic morphotropic phase boundary with tetragonal symmetry at room temperature, were the subject of this study. For the first time, 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 minor plane along the <110> direction