The structure of the disordered composite crystal (LaS)(1.18)VS2: The domain structure

A method for the crystal structure refinement of composite crystals with multiple domains of different symmetry is proposed and explained. For this a formula for the integral intensity of domain structures is given and a structure factor formula is derived. The method is applied to the disordered structure of the composite crystal (LaS)(1.18)VS2. The crystal consists of two subsystems each of which must be characterized introducing four types of domains. A further domain was introduced simulating all the diffraction intensities of other disordered layer sequences which contribute only to (Okl0) reflections. All reflections of the LaS subsystem can be indexed assuming lattice with a(I) = 5.705(2) Angstrom, b(I) = 5.830(2) Angstrom, c(I) = 66.89(1) Angstrom and alpha(I) = beta(I) = gamma(I) = 90 degrees and for the VS2 subsystem with a(II) = 3.366(1) Angstrom, b(II) = 5.830(1) Angstrom, c(II) = 44.59(1) Angstrom and alpha(II) = beta(II) = gamma(II) = 90 degrees. The refinement converged at a final R(F)-value of 0.153 (R-w(F) = 0.171) for 1112 reflections. Refinement of modulation amplitudes did not lead to reasonable bond distances and was therefore discarded. The results of the refinement of a non-modulated model are discussed.