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Characteristic features of polytypism in compounds with the La 18 W 10 O 57 ‐type structure
Author(s) -
Novikova Nataliya E.,
Sorokin Timofei A.,
Antipin Alexander M.,
Bolotidezhda B.,
Alekseeva Olga A.,
Sorokitaliya I.,
Voronkova Valentina I.
Publication year - 2019
Publication title -
acta crystallographica section c
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.304
H-Index - 17
ISSN - 2053-2296
DOI - 10.1107/s2053229619006107
Subject(s) - octahedron , crystallography , superstructure , crystal structure , tungsten , phase (matter) , atom (system on chip) , materials science , diffraction , type (biology) , chemistry , physics , geology , organic chemistry , computer science , optics , metallurgy , thermodynamics , embedded system , paleontology
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 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.