Crystal structure and twinning of Sb2AsS2, the synthetic analogue of paakkonenite P.BONAZn,D.Bo~M

The crystal structure of synthetic piilikkonenite, Sb2.o7Aso.93S2.oo, was determined. This study revealed that, in spite of the strong orthorhombic pseudosymmetry shown by Weissenberg photographs, the structure is monoclinic (space group C2/m), with a = 10.75(2), b = 3.959(3), c = 12.49(2) A, {1= 115.25(8)°, V = 481(1) A3, and Z = 4. The crystal structure was solved by direct methods using intensity data collected from a twinned crystal. The refinement led to an R index of 4.58%. The observed pseudosymmetry was shown to arise from {OO I} twinning. The structure can be described as a sequence of sheets stacked perpendicular to the a axis and mutually displaced by Ihb. Each sheet consists of Sb4S4As2ribbon-like units similar to the Sb4S6chains of stibnite but linked together by As-As bonds. The structural relationships with stibnite are discussed to explain the formation mechanism ofpaakkonenite and the presence of twinning. X-ray powder data for synthetic paakkonenite are also reported.