Tungsten Bronze-Type Solid Solutions Ba6-3xSm8+2xTi18O54(x=0.3, 0.5, 0.67, 0.71) with Superstructure

Structural parameters of tungsten bronze-type compounds Ba6-3xSm8+2xTi18O54(x=0.3, 0.5, 0.67, and 0.71) were refined in a space groupPbnmby the Rietveld method using high-resolution synchrotron radiation powder diffraction data. All the Aspecimens crystallized in the orthorhombic system with a superstructure having the doubledcaxis of the reported orthorhombic tungsten bronze-type structure. Unit-cell parameters (in ?) area=12.1715(5),b=22.3772(3),c=7.67523(9) forx=0.3,a=12.1568(1),b=22.3253(2),c=7.66301(7) forx=0.5,a=12.1472(1),b=22.2972(2),c=7.65338(6) forx=0.67, anda=12.1452(1),b=22.3029(2),c=7.65007(6) forx=0.71, andZ=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 withx= 0.3 and 0.5, pentagonal tunnel sites were fully occupied by Ba, and the remaining Ba partially substituted Sm at two of five tetragonal tunnel sites. No evidence was found for the substitution of Sm for Ba at the pentagonal sites, and thus vacancies Awere formed at the pentagonal sites forx=0.71. An amplitude of periodic modulation along thecaxis, which caused Asuperstructure, was increased with increasingxin the compositional range examined. A relationship between observed and calculated intensities of superstructure reflections suggests the presence of local lattice distortion, which takes its minimum atx=0.67.