Structural study of Al₇₈Mn_17.5Pt_4.5 and (re)constitution of the Al–Mn–Pt system in its vicinity

The previously reported Al–Mn–Pt X-phase with an unidentified structure (B. Grushko, J. Alloys Comp. 792 (2019) 1223) was concluded to be ternary extension of the binary orthorhombic Al–Mn T-phase. The geometry of the T-phase region in the Al–Mn–Pt phase diagram was updated and compared to the equivalent regions in other Al–Mn–(TM) systems (TM = Cr, Fe, Co, Ni, Cu, Pd). The lattice parameters of the Al₇₈Mn_17.5Pt_4.5 T-phase, corresponding to the high-Pt limit of its compositional region, were found to be a = 14.720(4) Å, b = 12.628(2) Å, c = 12.545(3) Å. The space group at this ternary composition was proved to be non-centrosymmetric Pna2₁, instead of Pnam - which describes the symmetry of the binary Al-Mn T-phase. At the Al₇₈Mn_17.5Pt_4.5 composition, the Pt atoms are not distributed randomly in the Mn/Al sublattices, but adopt two specific Wyckoff sites, therefore, this structure should be regarded as an ordered variant of the T-structure. A partial atomic model of the Al₇₈Mn_17.5Pt_4.5 T-phase containing 8 Pt, 24 Mn and 36 (out of 124 expected) Al atoms was derived using electron diffraction tomography data. The positions of the heavy atoms match those proposed for the non-centrosymmetric model of the T–phase, which was previously disputed. Nanometric precipitates of the R-phase with the plate-like shape were identified in the ternary T-phase grains.