Crystal structure of the Al78Mn17.5Pt4.5 phase as revealed by electron crystallography

Structure of high temperature “Al3Mn” (T) phase was investigated numerously. Studies of binary and ternary extensions of T-phase resulted in many published atomic models [1-8]. Until today, exact space group and atomic positions of transition metals in this structure is a matter of dispute. In current research, atomic model of the Al78Mn17.5Pt4.5 phase (quenched from 800 °C) was successfully derived using a combination of electron crystallography methods. This structure was regarded as ternary extension of the “Al3Mn” T–phase. The lattice parameters of the Al78Mn17.5Pt4.5 T-phase were found to be a = 14.720(4) Å, b = 12.628(2) Å, c = 12.545(3) Å (as refined against X-ray diffraction data). Using convergent beam electron diffraction (CBED), the space group of this ternary composition was proved to be non-centrosymmetric Pna21, instead of Pnam which describes the symmetry of the binary Tphase. Atomic model was determined applying direct methods, utilized in SIR2011 [9], on electron diffraction tomography data and refined using ShelXL [10]. At the Al78Mn17.5Pt4.5 composition, the Pt atoms were not distributed randomly in the Mn/Al sublattices, but adopted two specific Wyckoff sites, therefore, this composition should be regarded as an ordered variant of the T-structure. On the other hand, CBED study of the T-phase samples with a bit different stoichiometry (Al71.3Mn25.1Pt3.6) allowed attribution of the structure to the original T-phase structure type, i.e. centrosymmetric. Using Barnighausen tree [11], these two structures (centrosymmetric and non-centrosymmetric) were found to be related. 1. M. A. Taylor, The space group of MnAl3, Acta Cryst. 14(1) (1961) 84. https://doi.org/10.1107/S0365110X61000346