Growth, X-ray Diffraction and Dielectric Characterization of Pb5WO8 Single Crystals in the PbO–WO3 System

Single crystals of the Pb5WO8 phase were grown in the PbO–WO3 system by crystallization of (1 – x)PbO·xWO3 (x = 0.15–0.20) mixed melts. Thermogravimetric, X-ray diffraction, and dielectric studies of the single crystals were carried out. The phase melts at 712°С with decomposition to PbO and a liquid. The Pb5WO8 crystal structure is monoclinic (space group P21/n, 293 K) with the unit cell parameters a = 7.4430(1) Å, b = 12.1156(2) Å, c = 10.6284(2) Å, β = 90.658(1)°. The Pb5WO8 structure is retained at 100 K; the minor alterations in unit cell parameters are associated only with thermal expansion. The Pb5WO8 structure has a pronounced layered character; it appears as an alternation of layers formed of WO6 octahedra and strongly distorted PbO4 and PbO5 polyhedra in the direction [010]. A detailed crystal-chemical analysis of the structure was carried out. An important role of the Pb lone pair in the formation of characteristic voids in the structure was noted. The temperature-dependent dielectric permittivity and dielectric loss tangent feature relaxation peaks associated with lead and oxygen vacancies in the structure.