Investigations of the “tungsten–bronze”-type (Ba1–αSrα)6–xLa8+2x/3Ti18O54 (0⩽x⩽3) solid solutions

A study of the Ba/La/Sr ordering in the (Ba1–αSrα)6–xLa8+2x/3Ti18O54 (0⩽x⩽3) solid solutions was carried out by X-ray diffractometry and dielectric measurements. The maximum of substitution is obtained for x=0 and α =0.33, i.e. for Ba4Sr2La8Ti18O54 composition. The crystal structure is closely related to the tungsten–bronze-type structure with 3×3 octahedral units of perovskite columns (orthorhombic symmetry, s.g. Pnma, a≈22.4 Å, b≈7.7 Å, c≈12.2 Å). The A2 sites (pentagonal channels) and the A1 sites (rhombic channels within the perovskite-like columns) are occupied by Ba and La respectively. Sr always occupies the particular rhombic A1′ sites located at the junction of two perovskite-like columns which can also accept excess Ba and La. The A2 and A1′ sites may be cation deficient. The structure of Ba4(Sr2La8)Ti18O54 with structural composition A24(A1′2A18)Ti18O54, is fully ordered. The thermal behaviour of the permittivity and, for some compositions, the occurrence of a strong dispersion in a wide temperature range, would likely be connected both to a cationic disorder (Sr, Ba and/or La) and vacancies in the rhombic A1′ sites.