Investigations of the "tungsten-bronze"-type (Ba1-alpha Sr alpha)(6-x)La8+2x/3Ti18O54 (0 <= x <= 3) solid solutions

A study of the Ba/La/Sr ordering in the (Ba1-alphaSralpha)(6-x)La8+2x,3Ti18 O54 (0 less than or equal tox less than or equal to3) solid solutions was c arried out by X-ray diffractometry and dielectric measurements. The maximum of substitution is obtained for x=0 and alpha =0.33, i.e. for Ba4Sr2La8Ti1 8O54 composition. The crystal structure is closely related to the tungsten- bronze-type structure with 3 x 3 octahedral units of perovskite columns (or thorhombic symmetry, s.g. Pnma, a approximate to 22.4 Angstrom, b approxima te to7.7 Angstrom, c approximate to 12.2 Angstrom). The A2 sites (pentagona l channels) and the Al sites (rhombic channels within the perovskite-like c olumns) are occupied by Ba and La respectively. Sr always occupies the part icular rhombic Al' sites located at the junction of two perovskite-like col umns which can also accept excess Ba and La. The A2 and Al' sites may be ca tion deficient. The structure of Ba-4(Sr2La6)Ti18O54 with structural compos ition A2(4)(Al'Al-2(8))Ti18O54, is fully ordered. The thermal behaviour of the permittivity and, for some compositions, the occurrence of a strong dis persion in a wide temperature range, would likely be connected both to a ca tionic disorder (Sr, Ba and/or La) and vacancies in the rhombic Al' sites. (C) 2001 Elsevier Science Ltd. All rights reserved.