Journal of Solid State Chemistry 142, 336 — 343 (1999) Article ID jssc.1998.8042, available online at http://www.idealibrary.com on Tungsten Bronze-Type Solid Solutions Ba 63x Sm 82x Ti 18 O 54 (x  0.3, 0.5, 0.67, 0.71) with Superstructure H. Okudera, H. Nakamura, H. Toraya, and H. Ohsato* Ceramics Research Laboratory, Nagoya Institute of Technology, Asahigaoka 10-6-29, Tajimi 507-0071, Japan; *Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Received May 27, 1998; accepted September 14, 1998 Structural parameters of tungsten bronze-type compounds Ba 63x Sm 82x Ti 18 O 54 (x 0.3, 0.5, 0.67, and 0.71) were refined in a space group Pbnm by the Rietveld method using high- resolution synchrotron radiation powder diffraction data. All the specimens crystallized in the orthorhombic system with a super- structure having the doubled c axis of the reported orthorhombic tungsten bronze-type structure. Unit-cell parameters (in A s ) are a  12.1715(5), b  22.3772(3), c  7.67523(9) for x  0.3, a  12.1568(1), b  22.3253(2), c  7.66301(7) for x  0.5, a  12.1472(1), b  22.2972(2), c  7.65338(6) for x  0.67, and a  12.1452(1), b  22.3029(2), c  7.65007(6) for x  0.71, and Z  2. The use of two wavelengths allowed the determination of cation and vacancy distributions over two pentagonal and five tetragonal tunnels in the structure. In the compounds with x  0.3 and 0.5, pentagonal tunnel sites were fully occupied by Ba, and the remaining Ba partially substituted Sm at two of five tetra- gonal tunnel sites. No evidence was found for the substitution of Sm for Ba at the pentagonal sites, and thus vacancies were formed at the pentagonal sites for x  0.71. An amplitude of periodic modulation along the c axis, which caused superstruc- ture, was increased with increasing x in the compositional range examined. A relationship between observed and calculated inten- sities of superstructure reflections suggests the presence of local lattice distortion, which takes its minimum at x  0.67.  1999 Academic Press INTRODUCTION Physical properties of Ba  R  Ti  O  (R"La, Nd, and Sm) series have been investigated for their device applications related to their excellent dielectric properties (1—4). Matveeva et al. (1) reported that Ba  Pr  Ti  O  (x"0.75) crystallized in an orthorhombic system. Although the presence of a superstructure doubling the c axis was found, they refined structural parameters by assuming an average cell with a noncentrosymmetric space group Pba2  To whom correspondence should be addressed. (No. 32). On the other hand, Ohsato et al. (2) confirmed the formation of solid solutions for R"Sm over the composi- tion range 0.34x40.7 and refined the structural para- meters in a centrosymmetric space group Pbam (No. 55). The basic structure, assumed in these refinements, is a tung- sten bronze-type framework structure formed by TiO  octahedra sharing all apices with each other, and is charac- terized by distorted tetragonal and pentagonal tunnels running parallel to the c axis. The structure with the noncentrosymmetric space group can be derived by intro- ducing slight displacements of atoms along the c axis in the centrosymmetric space group and both have basically the same atomic arrangement. The superstructure doubling the c axis of the average structure has been also reported for Sm with x"0.75 (5). The space group of this structure is either Pbn2  (No. 33) or Pbnm (No. 62) (5). Recent reinvesti- gations of the solid solutions for R"Sm suggested the presence of a superstructure even for x"0.5 and 0.71 (6, 7). If the structure belongs to the centrosymmetric space group Pbnm, there should be five crystallographically independent tetragonal tunnel sites [designated by using symbols A  (1) to A  (5)] and two pentagonal tunnel sites [A  (1) and A  (2)] in a unit cell. However, distributions of cations and va- cancies over these seven crystallographic sites have not yet been determined. In the Ba  Sm  Ti  O  system, Sm is expected to be located in the perovskite-like tetragonal cavity (A  site) and larger Ba in the pentagonal cavity (A  site): a scheme of cation distribution in this system has been proposed as (Sm  Ba    ) [Ba  ] Ti  O  (x (2/3) (Sm    ) [Ba    ] Ti  O  (x52/3), [1] where parentheses and square brackets designate A  and A  sites, respectively, and open squares designate vacancy (8). As can be seen in Eq. [1], this system has a distinctive point at x"2/3 with respect to cation distribution. Substi- tution of Sm with Ba at the A  site in x(2/3 and the formation of vacancy at the A  site in x'2/3 have been 336 0022-4596/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.