STRUCTURE OF MN8SN5

The title compound crystallizes as a slightly incommensurate modulatio n of the B8-type structure. In a basic NiAs structure, similar to 60% of the trigonal pyramidal interstices are filled with Mn atoms in an o rdered manner. The highest corresponding commensurate space group is P bnm (Pnma, no. 62) with the cell parameters a = 21.9114 (4), b = 7.600 3 (5), c = 5.5247 (5) Angstrom. The four-dimensional superspace group of the incommensurate structure is Cmcm(alpha 00)0s0 (no. 63.8), with the conventional setting Amam(00 gamma)0s0. The cell parameters for th is incommensurate cell are a = 382 (1), b = 7.600 (2), c = 5.525 (2) A ngstrom, q = [0.616 (5), 0, 0]. The structural refinements were carrie d out on a multiply twinned specimen. The R factors were 0.037 for the incommensurate refinement and 0.046 for one commensurate approximatio n. The refinements unambiguously show that the modulation is caused by the step-like modulation of one Mn site, which is accompanied by smal l displacive modulations of the basic lattice. The incommensurate natu re of the modulation is manifested in a slight splitting of fifth-orde r satellites, visible in electron diffraction.