The Al61.49Mn11.35Ni4 phase in the Al–Mn–Ni system

The Al61.49Mn11.35Ni4 phase was synthesized by high-temperature sintering and its crystal structure has been re-determined.


Structure description
The ternary Al-Mn-Ni alloy system contains a variety of phases with complex or even quasicrystalline structures, most of which are not completely determined. Phase equilibria in the Al-rich region of the Al-Mn-Ni alloy system have been investigated previously. In this regard, a ternary phase with composition close to Al 60 Mn 11 Ni 4 was reported as thermodynamically stable, crystallizing in space group Bbmm (nonconventional setting of space group Cmcm) with unit-cell parameters of a = 23. 8, b = 12.5, c = 7.55 Å (Raynor, 1944). Its chemical composition was determined to be Al 80.0 Mn 14.7 Ni 5.3 for the same sample. This phase was later denominated the R phase (Robinson, 1954). The derived crystal-structure model for the R phase had some ambiguities because at that time it was not possible to accurately model the deficiencies or the type of element for some of the atomic sites (Robinson, 1954). The R phase with similar composition/crystal structure has also been discovered in other systems, such as the T 3 phase in the Al-Mn-Zn system or the Al 20 Mn 3 Cu 2 phase (Damjanovic, 1961). It is interesting to note that the orthorhombic phase in the Al-Mn system is isostructural with data reports the R phase and in coexistence with the decagonal quasicrystal in a rapidly solidified Al-Mn alloy, implying it is inseparable from the formation of quasicrystals (Li & Kuo, 1992).
In the present study, a slightly different crystal-structure model for the R phase in the Al-Mn-Ni system has been refined on basis of single-crystal X-ray diffraction data. This phase has similar unit-cell parameters to the previously reported R phase (Table 1, using the conventional setting Cmcm). Its chemical composition was refined to be Al 61.49 Mn 11.35 Ni 4 , in accordance with complementary SEM/EDX results (see Fig. S1 and Table S1 of the supporting information).
In comparison with the R phase, the R 0 phase has a slightly higher Al and Mn content. A detailed comparison of the atomic labelling and coordinates between these two structure models along with the transformation matrix that transforms the original non-conventional setting to the current standard setting can be found in Table S2 of the supporting information. The R 0 phase has two reversed sites compared to the original R phase whereby Mn4 in the original model becomes Ni1 in the current model, and vice versa. In addition, the R 0 phase shows positional disorder of one Al site (Al7), and one Mn site (Mn2) with partial occupancy. Fig. 1 shows the distribution of all atoms in the unit cell of Al 61.49 Mn 11.35 Ni 4 with four distorted icosahedra illustrated for simplicity. The environments of the Mn3 and Mn4 sites are shown in Fig. 2a and 2b, respectively. The icosahedron centered at Mn3 is surrounded solely by Al atoms (Al3, Al4, Al5, Al6, Al10, Al11, Al12 and Al13) while that centered at Mn4 atom is composed by eleven Al atoms (Al1, Al2, Al4, Al5, Al9, Al11 and Al12) and one Mn atom (Mn4); all of the corresponding atomic sites are fully occupied. The polyhedron centered at Al3 is composed of a pentagonal prism capped by two atoms at the base faces, as shown in Fig. 3a. The environments of Al3 are displayed in Fig. 3b, where ten Al atoms (Al6, Al12 and Al13) and two Mn atoms (Mn3) surround the central atom.

Synthesis and crystallization
The high-purity elements Al (indicated purity 99.8%; 2.4285 g), Mn (indicated purity 99.96%; 0.5768 g) and Ni (indicated purity 99.9%; 0.2641 g) were mixed in the molar ratio 60:7:3 and ground in an agate mortar. The blended powders were placed into a cemented carbide grinding mound of 9.6 mm diameter and pressed at 4 MPa for about 5 min. The obtained cylindrical block was put into a silica glass tube and vacuum-sealed by a home-made sealing machine. The resulting ampoule then was placed in a furnace (SG-XQL1200) and heated up to 473 K for 10 min with a heating rate of 10 K min À1 and then heated up to 1373 K for 30 min with the same heating rate. Finally, the sample was slowly cooled to room temperature by turning off the furnace power. Suitable pieces of single-crystal grains were broken and selected from the product for single-crystal X-ray diffraction.

Figure 1
The crystal structure of Al 61.49 Mn 11.35 Ni 4 with two Mn3 atoms and two Mn4 atoms displayed with their coordination environments as polyhedra.
occupied, and its site occupation factor (s.o.f.) was refined to 0.677 (5). The aluminium site Al17 was found to be disordered over two positions with refined s.o.f.s of 0.811 (8) and 0.121 (7) for Al7A and Al7B, respectively. The same anisotropic displacement parameters were used for these two split Al sites. All Ni sites in the present model show full occupancy. The maximum and minimum residual electron densities in the final difference map are located 1.42 Å from site Al11 and 0.57 Å from site Al7A, respectively. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.85 e Å −3 Δρ min = −1.03 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.