A commensurately modulated structure of parabutlerite, Fe~IIISO_4(OH)·2H_2O

摘要

Parabutlerite, orthorhombic Fe~IIISO_4(OH)·2H_2O, has been reinvestigated using single-crystal X-ray diffraction. The structure of parabutlerite is commensurately modulated, with a = 20.0789 (8), b = 7.4024 (7), c = 7.2294 (15) ? and q = 0.4b*. The superstructure has been determined, using a superspace approach, as having the superspace group Pnma(0β0)s0s and t_0 = 1/20, and refined to R = 0.0295 for 2392 main reflections with I > 3σ(I). The structure consists of infinite chains of Fe octahedra that are linked via vertices (OH groups); these chains are encased from both sides by SO_4 tetrahedra. The displacive modulation of atoms in parabutlerite is connected with a tilt of the chains around the b axis towards the adjacent chains due to the accommodation of an energetically more favorable hydrogen-bond geometry.