(3 + 2)-Dimensional superspace approach to the structure of the incommensurate intergrowth compound: (SbS)1.15TiS2

Abstract

The inorganic misfit-layer compound (SbS)(1.15)TiS2 was prepared by high-temperature reaction of the elements. The structure, determined by single-crystal X-ray diffraction, is described by two interpenetrating incommensurately modulated subsystems. The first subsystem comprises TiS2 sandwiches, with Ti atoms in trigonal-antiprisms of S atoms. The lattice parameters are alpha'(11) = 3.403 (1), alpha'(12) = 3.410 (1), alpha(13)' = 11.385 (1) Angstrom, alpha'(11) = 81.544 (7), beta'(1) = 82.817 (8) and gamma'(1) = 60.08 (1)degrees. The second subsystem is built of intrinsically interface modulated double layers of SbS. The basic structure unit-cell dimensions are given by alpha'(21) = 2.954 (1), alpha'(22) = 2.968 (1), alpha'(23) = 11.311 (1) Angstrom, alpha(2) = 83.973 (8), beta(2) = 85.87 (1), gamma(2) = 84.06 (1)degrees. The interface modulation wavevector of SbS is given by q = 0.409(a*'(21) + a*'(22)). The two subsystems have the common (a*'(v2), a*'(v3)) plane. The whole X-ray diffraction pattern is indexed with five integer indices, thus a (3 + 2)-dimensional superspace group is used to analyse the complete structure. Both the superspace-group symmetry and the subsystem symmetries are centrosymmetric triclinic, belonging to the superspace group P $($) over bar$$ 1. Refinement on 2483 reflections with I > 2.5 sigma(I) converged to wR = 0.069 (R = 0.062). The final structure model consists of both occupational and displacive modulations for the atoms in the SbS subsystem, which results in zigzag clusters of -Sb-Sb and -S-S- parallel to the [1 $($) over bar$$ 10] direction of SbS. The shortest Sb-Sb and S-S distances are 2.84 and 3.43 Angstrom, respectively. The average valence of Sb was calculated as 3.13. The distinguishing feature is the incommensurate ordering of Sb/S atoms on the rocksalt structure, with a modulation wavevector apparently unrelated to the periodicities of the TiS2 subsystem.