Local atomic order in sodium p-chlorobenzenesulfonate monohydrate studied by pair distribution function analyses and lattice-energy minimisations

Abstract

Routine X-ray single-crystal structure analysis of Na[C₆H₄ClO₃S]·H₂O resulted in an orthorhombic layer structure with space group Pnma, Z=4. In this crystal structure the phenyl rings in a single layer are disordered over two mutually perpendicular orientations with occupancies of 0.500(4) each. Structure determination including superstructure reflections and irregular shaped reflections revealed a twinned, ordered, monoclinic structure, P2₁/c, Z=8. Here, within a layer the phenyl rings are arranged alternatingly perpendicular. Pair distribution function analysis (PDF) and lattice-energy calculations confirmed this arrangement. Faint diffuse scattering streaks point to a one-dimensional disorder (concerning the layer-stacking sequence). Application of the order-disorder (OD) approach led to three polytype structures with maximum degree of order (MDO); one of them is the experimental monoclinic structure. This structure was found to have the most favourable energy in the lattice-energy minimisations. However, the energy differences between different polytypes with different stacking sequences are rather small. Hence, the lattice-energy minimisations confirmed the monoclinic P2₁/c structure with stacking disorder.