REFINEMENT OF STRUCTURES OF THE COMPOSITE CRYSTALS BAXFE2S4 (X = 10/9 AND 9/8) IN 4-DIMENSIONAL FORMALISM

The structures of the composite crystals of tetragonal columnar sulfide, Ba10/9Fe2S4 (M(r) = 392.55) and Ba9/8Fe2S4 (M(r) = 394.46), have been refined on the basis of a four-dimensional superspace group using the X-ray diffraction data collected by Grey [Acta Cryst. (1975), B31, 45-48] and Hoggins & Steinfink [Acta Cryst. (1977), B33, 673-678]. The crystals are composed of linear strings of edge-shared FeS4 tetrahedra (FeS4/2) infinity and rows of Ba ions. R(F) was 0.082 on a commensurate modulated structure model with 23 structural parameters using 212 reflections for Ba10/9Fe2S4, and 0.153 with 33 parameters using 1083 reflections for Ba9/8Fe2S4. The symmetry operations of the superspace group employed are (0,0,0,0; 1/2,1/2,1/2,1/2) + x1,x2,x3,x4; -x2,x1,x3,x4; -x1,x2,1/2 + x3,x4; x2,x1,1/2 + x3,x4; -x1, -x2,x3,x4; x2, -x1,x3,x4; x1, -x2,1/2 + x3,x4; -x2, -x1,1/2 + x3,x4, and the superspace group can be transformed into P1ss(I4bm) with a basis transformation. The unit cells and other crystal data are a1 = a2 = 7.776, a3 = 5.540 angstrom, sigma = (0010/9), V = 334.98 angstrom 3, D(x) = 3.89 Mg m-3, Z = 2 for Ba10/9-Fe2S4, and a1 = a2 = 7.776, a3 = 5.551 angstrom, sigma = (009/8), V = 334.10 angstrom 3, D(x) = 3.90 Mg m-3, Z = 2 for Ba9/8Fe2S4.