Determination of the modulated structure of the inorganic misfit layer compound (PbS)1.18TiS2

Abstract

Single-crystal X-ray diffraction results (Mo K-alphaBAR radiation, lambda = 0.71073 angstrom) are presented for the inorganic misfit layer compound titanium sulfide (PbS)1.18TiS2 which can be described as a two-component structure. The first subsystem (TiS2, nu = 1) has space-group symmetry C2(1)/m, and a basic structure unit cell given by a11 = 3.409 (1), a12 = 5.880 (2), a13 = 11.760 (2) angstrom and alpha-1 = 95.29 (2)-degrees. The modulation wavevector is q1 = a21* = alpha-a11*, with alpha = 0.5878 (3). Its subsystem superspace group is [GRAPHICS] (alpha, 0, 0). The second subsystem (PbS, nu = 2) has space group C2/m and a basic structure unit cell given by a21 = 5.800 (2), a22 = 5.881 (2), a23 = 11.759 (2) angstrom and alpha-2 = 95.27 (2)-degrees. The modulation wavevector is q2 = a11*. The subsystem superspace group is [GRAPHICS] (alpha-1, 0, 0). The relation between the two unit cells is defined by the common (a-nu-2*, a-nu-3*) plane. The symmetry of the complete system is described by the single superspace group G(s) = [GRAPHICS] (alpha, 0, 0). Reciprocal lattice parameters for this superspace embedding are a1* = a11*, a2* = a12*, a3* = a13* and a4* = a21*. Refinements on 1449 main reflections, with I > 2.5-sigma(I), converged smoothly to R(F2) = 0.064 (R(F) = 0.069). The final structure model included displacive modulation parameters up to second harmonics for Pb and first harmonics for the other atoms. The largest modulation amplitudes are on both atoms of the PbS subsystem. They mainly desribe displacements parallel to the layers, along the commensurate direction a-nu-2. A detailed analysis is given of the coordination of the Pb (nu = 2) and S (nu = 1) atoms by plotting interatomic distances as a function of the fourth superspace coordinate.