Three NiAs-Ni2In type structures in the Mn-Sn system

The B8-type structure field of the MnSn system has been investigated. Two high temperature phases (HTP1 and HTP2) and one low temperature phase (Mn3Sn2) were found. They all crystallize with the NiAs structure type with part of the trigonal bipyramidal interstices filed by manganese atoms in an ordered manner. The ordering as well as the manganese content is different for the three phases, giving rise to three different orthorhombic superstructures. Mn3Sn2 seems to have the lowest manganese content, since the corresponding basal unit cell is smaller than for HTP1-2. Structural models of the phases are based on selected area electron diffraction, X-ray powder diffraction, and preliminary single crystal X-ray measurements. The ideal cell parameters found are (a = 7a(hex), b = root 3a(hex), c = c(hex)), (a = 5a(hex), b = root 3a(hex), c = c(hex)), and (a = 2a(hex), b = root 3a(hex), c = c(hex)) for HTP1, HTP2, and Mn3Sn2, respectively. The crystal structure of Mn3Sn2 has been refined by means of the Rietveld method from X-ray powder diffraction data. Mn3Sn2 is orthorhombic, Pnma, a = 7.5547(2), b = 5.4994(2), c = 8.5842(2) Angstrom, Z = 4. (Pbnm in the setting above.) The compound is isostructural with Ni3Sn2 and gamma'-Co3Sn2 (H. Fjellvag and A. Kjekshus, Acta Chem. Scand. A 40, 23-30 (1986)). Final R-p = 8.97%, R-wp = 11.44%, GOF = 2.86, and R-Bragg = 4.11% using 43 parameters and 5701 observations and 330 Bragg reflections. (C) 1997 Academic Press.