High pressure polymorphism of rare earth sulfideiodides

High pressure transformations of rare earth sulfideiodides are reported. Compounds with the GdSI-type structure (GdSI to DySI) transform to the SmSI-type within the pressure range up to 40 kb. For GdSI itself an α-CeSI-type arrangement can be achieved additionally in the higher pressure range. All compounds crystallizing in the SmSI-type at ambient pressures transform to the α-CeSI-type under such conditions. We explain these structures briefly in terms of a topological description, and we offer a structure model for GdSI. Possible reaction mechanism are discussed in comparison with similar transformations in LnOCl and LnSF compounds. (author)