AN X-RAY AND ELECTRON-DIFFRACTION STUDY OF THE TETRAGONAL LEAD TUNGSTEN BRONZE PB0.26WO3

The average structure of the tetragonal lead tungsten bronze, Pb0.26WO 3, has been determined by single-crystal X-ray diffraction (P4/mbm spa ce group, a(av) = 12.217(1) Angstrom, c(av) = 3.7828(7) Angstrom and Z = 10). The structure refinement based on 370 observed unique reflecti ons (2539 totally measured reflections) with F-0 > 4 sigma(F-0) conver ged to R = 0.035 (R-w = 0.031). All but one of the oxygen atoms exhibi t a twofold disorder. The lead atoms are distributed over three, symme trically independent, positions inside the pentagonal tunnels. Electro n diffraction observations recognize the X-ray average structure. Furt hermore, satellite spots appear an the ED patterns revealing a modulat ed structure, which may be described in terms of a periodic repetition of antiphase boundary planes inside a superstructure related to the a verage structure by a(s) = root 2a(av) and c(s) = 2c(av). The repetiti on distance is d = 5/2a(s) and the displacement vector is R = 1/2 a(av ). (C) 1997 Academic Press.