X-ray and electron diffraction study of sartorite. A periodic antiphase boundary structure and polymorphism.

Abstract

A number of sartorite specimens from Binnathal have been examined by single-crystal X-ray and electron diffractions. The substructure reflections of any crystal picked out from those specimens are consistent with the space group Pbnm and have the subcell with a∼19.6, b∼7.9 and c∼4.2A as reported to date. The reported peculiar mode in diffraction-intensity distribution that it varies from one crystal to another or even from one portion to another in the same crystal has also been observed both in the single-crystal X-ray diffraction and in the selected-area electron diffraction. The mode of intensity distribution of the superstructure reflections, however, has been found to have two distinct types, one has a monoclinic symmetry and the other an orthorhombic symmetry. Crystalline phases showing the former type have been denoted ‘clinosartorite’ and those showing the latter ‘orthosartorite’. In addition to them there are a variety of disordered types as well. The complexity in diffraction patterns can be interpreted essentially as composites of the patterns from the coexisting two distinct types (clinosartorite is frequently twinned). The monoclinic phase has features suggesting a periodic antiphase boundary structure (APBs) with two-dimensional structural modulation. The relationship between the new monoclinic true cell, in which cell edges are chosen in accordance with the structural features, and the conventional cell is described as: aNR=3×aCS–11×cCS, bNR=bCS, cNR=11×cCS, where suffixes NR and CS indicate the newly-derived true cell and the conventional subcell, respectively. On the other hand, ‘orthosartorite’ has the same three cell edges as those reported for the conventional true cell, namely, 3×aCS, bCS, and 11×cCS. The periodic antiphasing and expected modulation of the structure would be rationalized, in terms of a mechanism of reducing lattice strain that may occur owing to the dimensional misfit between Pb–S and As–S links in the structure.