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

Abstract

Abstract A study of the Ba/La/Sr ordering in the (Ba 1– α Sr α ) 6– x La 8+2 x /3 Ti 18 O 54 (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 Ba 4 Sr 2 La 8 Ti 18 O 54 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 A, b ≈7.7 A, c ≈12.2 A). 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 Ba 4 (Sr 2 La 8 )Ti 18 O 54 with structural composition A2 4 (A1′ 2 A1 8 )Ti 18 O 54 , 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.