Crystal structure of Al2.95Cr0.59, a phase closely related to the η-phase in the binary Al–Cr system

Al2.95Cr0.59 was synthesized by high-pressure sintering (HPS). It has a slightly lower Al content than the closely related η-Al11Cr2 phase.

data reports crystal firstly reported by Lilienfeld et al. (1986) shortly after the discovery of quasicrystals in rapidly solidified Al-Mn alloys (Shechtman et al., 1984). Inoue et al. (1987) found that single-phase icosahedral quasicrystals have formed in the vicinity of about 15.4 at.% Cr in rapidly quenched Al-Cr alloys, and the quasicrystal can be approximately formulated to have the composition Al 11 Cr 2 . In terms of thermal stability, the quasicrystal with composition Al 84.6 Cr 15.4 decomposes into a stable orthorhombic Al 11 Cr 2 phase while the quasicrystal containing less Cr (6 to 14.5 at% Cr) changes directly to stable phases of Al + Al 7 Cr and Al 7 Cr + Al 11 Cr 2. Icosahedral quasicrystals from the Al-Cr alloy containing 7 to 15 at.% Cr have twinned Al 7 Cr as final decomposition product while the equilibrium -Al 11 Cr 2 phase is completely absent during the decomposition of quasicrystals (Swamy et al., 1989). Interestingly, Zhang et al. (1988) also found rotational twins of the Al 45 Cr 7 phase while no Al 11 Cr 2 phase was found in a rapidly solidified Al 7 Cr alloy. From these pioneering studies one can conclude that the Al 11 Cr 2 phase can coexist with the quasicrystalline phase(s). Therefore, it is pivotal to decipher the formation of the Al 11 Cr 2 phase in order to enhance our understanding of the formation of quasicrystals as well as precipitations in the Al-Cr binary system. For the present investigation, we used high-pressure sintering (HPS) of a stoichiometric Al:Cr mixture (molar ratio = 11:2) for crystal growth.
We have named the present phase 0 -Al 11 Cr 2 . Its crystal structure is closely related to the -Al 11 Cr 2 phase previously reported by Cao & Kuo (2008a), however with a different refined composition (Al:Cr ratio = 5.04). There are 66 Al and 14 Cr independent atomic positions and a total of 616 atoms (514 Al + 102 Cr) in the unit cell of 0 -Al 11 Cr 2 . The crystal structure of -Al 11 Cr 2 comprises the same total number of atoms but with 516 Al atoms and 100 Cr atoms. In the 0 -Al 11 Cr 2 phase, there are only five mixed-occupied sites by The crystal structure of 0 -Al 11 Cr 2 projected along [010].

Figure 1
Icosahedrally surrounded sites (indicated by circles) in the crystal structure of 0 -Al 11 Cr 2 in a projection along [101].
with a ratio of 0.8291:0.1709. A detailed comparison of coordinates and occupancies for related atoms in the crystal structure models of 0 -Al 11 Cr 2 and -Al 11 Cr 2 can be found in the supporting information (Table S1).
Since the crystal structure of -Al 11 Cr 2 was described in detail (Cao & Kuo, 2008a), we report here only the most important features. Figs. 1 and 2 illustrates the crystal structure of 0 -Al 11 Cr 2 in a projection along [101] and [010], respectively. In Fig. 1, icosahedrally surrounded sites (indicated by circles) can be seen. One of such icosahedra (here around Cr6) was selected to show its chemical environment (Fig. 3). It is quite interesting that there are no split sites for Al atoms in the present 0 -Al 11 Cr 2 structure model. Scanning electron microscope (SEM) micrographs and energy dispersive X-ray spectroscopy (EDS) analysis of a fragment from which single crystals were selected for X-ray diffraction studies revealed that theand 0 -Al 11 Cr 2 phases have a very similar (and based on this method indistinguishable) chemical composition (see Fig. S1 and Table S2 in the supporting information). However, the refined chemical composition of the present 0 -Al 11 Cr 2 phase using single-crystal X-ray analysis reveals that it has two Al atoms fewer and two Cr atoms more than the reported -Al 11 Cr 2 phase.

Synthesis and crystallization
The high-purity elements Al (indicated purity 99.8%; 1.2537 g) and Cr (indicated purity 99.95%; 0.4389 g) were mixed in the stoichiometric ratio 11:2 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. A uniformly cylindrical block with 9.6 mm in diameter and 10.0 mm in height was obtained that was subsequently loaded into a six-anvil high-temperature high-pressure apparatus as described elsewhere (Xia et al., 2018). For the present high-pressure sintering experiments, the sample was pressurized up to 5 GPa and heated up to 1222 K for 30 minutes, slowly cooled to 1092 K and held at this temperature for 2 h, and then cooled to room temperature by turning off the furnace power. Suitable pieces of single-crystal grains were selected from the products for X-ray diffraction experiments. 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.