STRUCTURE OF THE 1-DIMENSIONALLY DISORDER ED COMPOSITE CRYSTAL (LAS)(1,18)VS2

The one-dimensionally disordered structure of the composite crystal of synthetic (LaS)(1.18)VS2 has been determined based on the X-ray diffr action intensities collected by using monochromatized synchrotron radi ation of wavelength 1.180 Angstrom. The ordered (100) projection was r efined based on 94 unique intensities collected on a diffractometer us ing MoKalpha radiation; R = 0.067, R(w) = 0.084. The structure consist s of interleaved layers of LaS and VS2, each of them forming a substru cture which can be described based on a C-centred orthorhombic lattice with a(1) = 5.705(1) Angstrom, b = 5.828(1) Angstrom, c = 22.32(2) An gstrom and Z = 8 for LaS and a(2) = 3.366(3) Angstrom, b = 5.828(1) An gstrom, c = 22.32(2) Angstrom and Z = 4 for VS2. The b and c axes are common to both substructures. D-x = 4.806 Mg m(-3). The structure of t he (100) projection is isotypic with that of the orthorhombic (PbS)(1. 18)TiS2; plane group pgm, b' = b/2, c' = c. The La and S atoms of LaS are arranged like Na and Cl in the NaCl structure. The V atom is surro unded by six S atoms octahedrally. The LaS layers are related to each other through a twofold screw axis parallel to [100] with a screw tran slation of +/-0.32a(1), while the nearest VS2 layers are coupled by a glide plane perpendicular to [001] with a glide translation of +/-0.25 a(2). Stochastic change of the signs causes a one-dimensional disorder .