THE STRUCTURE OF THE DISORDERED COMPOSITE CRYSTAL (LAS)(1.18)VS2 - THE DOMAIN-STRUCTURE

A method for the crystal structure refinement of composite crystals wi th multiple domains of different symmetry is proposed and explained. F or this a formula for the integral intensity of domain structures is g iven and a structure factor formula is derived. The method is applied to the disordered structure of the composite crystal (LaS)(1.18)VS2. T he crystal consists of two subsystems each of which must be characteri zed 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) = 5.70 5(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 w ith a(II) = 3.366(1) Angstrom, b(II) = 5.830(1) Angstrom, c(II) = 44.5 9(1) Angstrom and alpha(II) = beta(II) = gamma(II) = 90 degrees. The r efinement converged at a final R(F)-value of 0.153 (R-w(F) = 0.171) fo r 1112 reflections. Refinement of modulation amplitudes did not lead t o reasonable bond distances and was therefore discarded. The results o f the refinement of a non-modulated model are discussed.