Structure of a polytype of the inorganic misfit-layer compound (PbS)1.18TiS2.

Abstract

A second modification is reported of the inorganic misfit-layer compound (PbS)1.18TiS2. It is found to be a true polytype of the previously analyzed monoclinic form. The AB stacking of layers in the latter is replaced by a stacking AB A'B' in this orthorhombic modification. Single-crystal x-ray diffraction measurements (Mo K-alpha radiation, lambda=0.71073 angstrom) were performed on a crystal of (PbS)1.18TiS2. The structure is described by two modulated subsystems (nu = 1,2), which have the (a(nu-2)*,a(nu-3)*) reciprocal-lattice plane in common. A superspace embedding follows from four reciprocal-lattice vectors, given by a1*=a11*, a2*=a12*, a3*=a13*, and a4*=a21*. The symmetry is given by the super space group G(s)=P:Cmca(alpha-0,0,)1ssBAR. Lattice parameters for the first subsystem (TiS2) are a11 = 3.409(1) angstrom, a12 = 5.882(1) angstrom, and a13=23.409(3) angstrom, with modulation wave vector q1=a21*=alpha-oa11*, and alpha-0 = 0.5875(3). Its subsystem superspace group is P:Cmca(alpha-0,0,0)1ssBAR. The second subsystem (PbS) has lattice parameters a21=5.802(1) angstrom, a22=a12, and a23=a13, with modulation wave vector q2=a11*. The subsystem superspace group is P:Cmna(alpha-0(-1),0,0)11sBAR. Refinements on 1123 main reflections and 186 first-order satellites with I > 2.5-sigma(I), converged smoothly to R(F)2=0.163(R(F)=0.111). 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 found on both atoms of the PbS subsystem. They mainly describe displacements parallel to the layers, along the commensurate direction a(nu-2). It is shown that the local structures in the monoclinic and the orthorhombic forms are very much alike.