Characteristic features of polytypism in compounds with the La18W10O57-type structure

Crystals with the La 18 W 10 O 57 -type structure (6H and 5H polytypes) were obtained by a self-flux method from high-temperature solutions. Some of the crystal samples were studied by single-crystal X-ray structure analysis. The diffraction patterns indicated that two phases co-exist in each sample. The hexagonal lattices have a common period of a ≈ 9.0 Å and are non-equal in length but have equally oriented superstructure periods 6 c (phase I) and 5 c (phase II), c ≈ 5.4 Å. The structures of phases I and II were solved in the symmetry groups P \overline{6}2 c and P 321, respectively, based on the X-ray data for crystals I and II, with predominant content of the first and second phase. The motif of isolated WO 6 prisms with W atoms on the cell edges is common to both phases. WO 6 octahedra, both isolated and joined by faces, are distributed along the c axis within the unit cells. Phase I contains extra layers of isolated WO 6 octahedra compared to phase II. Tungsten sites in joined octahedra are disordered and partially occupied. Disordering is more expressed in phase II, which in return contains rather more W and O per atom of La. The refined chemical compositions are La 18 W 10 O 57 for I and La 15 W 8.5 O 48 for II.