SOLVING CRYSTAL SUPERSTRUCTURES BY THE DIRECT METHOD

Superstructures are often observed in inorganic crystals, including many minerals and certain important solid state materials. In diffraction patterns of such crystals, superstructure reflections, which carry information of the structure details, are generally very weak. The common practice in structure determination is, therefore, to ignore the superstructure reflections at first, making use only of the strong pseudos-tructure reflections to find a pseudostructure to account for the essential structural features. The precise superstructure is then derived on its basis. However, this approach is complicated in consequence of the impossibility to derive the latter directly from the former by means of ordinary techniques of structure refinement. To overcome this difficulty, it is proposed in this paper that the phases of superstructure reflections can be derived directly from those of the pseudostructure reflections. This considerably simplifies the procedure of superstructure determination. Applications to practical structures confirmed the effectiveness of the method.