Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Chemistry and Pharmacology
The title compound crystallizes as a slightly incommensurate modulation of the B8-type structure. In a basic NiAs structure, ∼60% of the trigonal pyramidal interstices are filled with Mn atoms in an ordered manner. The highest corresponding commensurate space group is Pbnm (Pnma, No. 62) with the cell parameters a = 21.9114 (4), b: 7.6003 (5), c = 5.5247 (5) Å. The four-dimensional superspace group of the incommensurate structure is Cmcm(α00)0s0 (No. 63.8), with the conventional setting Amam(00γ)0s0. The cell parameters for this incommensurate cell are a = 382 (1), b = 7.600 (2), c = 5.525 (2) Å, q = [0.616 (5), 0, 0]. The structural refinements were carried out on a multiply twinned specimen. The R-factors were 0.037 for the incommensurate refinement and 0.046 for one commensurate approximation. The refinements unambiguously show that the modulation is caused by the step-like modulation of one Mn site, which is accompanied by small displacive modulations of the basic lattice. The incommensurate nature of the modulation is manifested in a slight splitting of fifth-order satellites, visible in electron diffraction.
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