Abstract
Binary high temperature “Al3Mn” (T-phase) and its extensions in ternary systems were the subjects of numerous crystallographic investigations. The results were ambiguous regarding the existence or lack of the center of symmetry: both Pna21 and Pnam space groups were reported. Our research on the Al–Mn–Pt T-phase allowed concluding that inside a continuous homogeneity region, the structure of the Al-rich T-phase (e.g., Al78Mn17.5Pt4.5) belongs to the non-centrosymmetric Pna21 space group, while the structure of the Al-poor T-phase (such as Al71.3Mn25.1Pt3.6) is centrosymmetric, i.e., Pnam. Following metallurgical and crystallographic considerations, the change in the symmetry was explained.
Highlights
The Al–Mn so-called T-phase, known as high-temperature “Al3Mn”, is formed in a compositional region between ~25 and 29 at. % Mn and a temperature range of 895–1002 ◦C
The compositional region of the T-phase widely extends in numerous ternary Al–MnM alloy systems, where M is transition metal, which results in a significant decrease in its stability temperature
In 1993, its atomic model was proposed again in the framework of the Pnam space group [6], while in 1994, Shi et al [7] insisted on Pna21
Summary
The Al–Mn so-called T-phase, known as high-temperature “Al3Mn”, is formed in a compositional region between ~25 and 29 at. % Mn and a temperature range of 895–1002 ◦C (see [1] and references therein). The compositional region of the T-phase widely extends in numerous ternary Al–MnM alloy systems, where M is transition metal (see [2] and references therein), which results in a significant decrease in its stability temperature. In 1961, the centrosymmetric Pnam space group was chosen to describe its symmetry [4], while in 1992, in a series of publications by Li et al (see [5] and references therein), the non-centrosymmetric Pna was argued. In 1995, Pavlyuk et al [8] succeeded in refining T phase’s structure in the framework of the Pnam space group but failed when using Pna, which, according to these authors, would not give significantly better results. The major discrepancies were found in the occupancies and positions of transition metals
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