Crystal structure of AlFe0.95

Three Fe-deficient B2-type AlFe1 – δ phases have been synthesized, and the exact crystal structure obtained from single-crystal X-ray diffraction measurements is reported for δ = 0.05.

Three B2-type intermetallic AlFe 1 -� phases (0.18 < � < 0.05) in the Al-Fe binary system were synthesized by smelting and high temperature sintering methods.The exact crystal structure for � = 0.05 was refined by single-crystal X-ray diffraction.The amount of vacancy defects at the Fe atom sites was obtained by refining the corresponding site occupancy factor, converging to the chemical formula AlFe 0.95 , with a structure identical to that of ideal AlFe models inferred from powder X-ray or neutron diffraction patterns.

Structure description
There are diverse intermetallic phases in the Al-Fe system, among which AlFe has attracted much attention because of its special B2 structure.For example, Van der Kraan & Buschow (1986) studied the crystal structure of the AlFe phase, after heat treatment at 1273 K for 50 h, by X-ray powder diffraction.The authors suggested that AlFe has a CsCl-type structure with cell parameter a = 2.907 A ˚.During the study of the crystal structure of La(T,Al) 13 (T = Fe, Co), a coexisting AlFe cubic phase was discovered.The crystal structure of cubic AlFe was also refined using X-ray powder diffraction data, affording the cell parameter a = 2.889 (4) A ˚, and a model in which Al and Fe atoms are occupying the Wyckoff positions 1a and 1b, respectively, in space group Pm3m (Guo et al., 1997).Makhlouf et al. (1994) studied the structure of the magnetic alloys FeAl 1 -x Rh x by X-ray diffraction, and concluded that the crystal structure of these alloys remains in the B2 structural type.Stein et al. (2010), using the high-temperature neutron diffraction approach, found that the cell parameter of the AlFe phase gradually increases by increasing the temperature: the cell parameter of the AlFe phase at room temperature, 373, 1353 and 1393 K, is 2.9097, 2.9136, 2.9681 and 2.9720 A ˚, respectively.They also proposed that the AlFe phase has a B2-type crystal structure (Pm3m space group, cP2 Pearson symbol).
In the present work, three kinds of Fe-deficient B2-type AlFe 1-� phases were synthesized by smelting and hightemperature sintering methods, with very similar lattice parameters.The AlFe 0.95 (� = 0.05) phase was obtained by the smelting method, while AlFe 0.82 and AlFe 0.84 phases (� = 0.18 and � = 0.16) were obtained from an intergrowth sample by the high-temperature sintering method.The refined chemical formula of the AlFe 0.95 phase is in accordance with the complementary EDX results (see Table S1 of the supporting information).Different options for refinements are listed in Table S2 of the supporting information.The structure description reported herein is for the AlFe 0.95 (� = 0.05) phase.
Fig. 1 shows the unit cell of AlFe 0.95 .The environments of the Al and Fe sites are shown in Figs. 2 and 3, respectively.The Al1 atom at (0, 0, 0) is centred at a rhombic dodecahedron, whose vertices are six Al1 atoms and eight Fe1 atoms; conversely, the Fe1 site at (1/2, 1/2, 1/2) is surrounded by eight Al1 atoms and six Fe1 atoms.The shortest Al1 to Fe1 separation is 2.5164 (4) A ˚and the shortest Al1 to Al1 link is 2.9057 (5) A ˚.The R 1 refinement residue versus � values has been plotted for 0 < � < 0.1 and is shown in Fig. 4, where one can see that R 1 has the lowest value when the chemical occupancy of Fe atoms is 0.95.

Synthesis and crystallization
For the here reported sample obtained by smelting (� = 0.05), high-purity elements Al (indicated purity 99.95%; 1.629 g) and Fe (indicated purity 99.99%; 3.371 g) were mixed in the stoichiometric ratio 1:1 and the alloy was prepared from the elements by arc melting under an argon atmosphere.Suitable pieces of single-crystal grains were broken and selected from the product for single-crystal X-ray diffraction.
For the sample obtained by high-temperature sintering (� = 0.16 and 0.18), high-purity elements Al (indicated purity 99.95%; 0.7362 g) and Fe (indicated purity 99.9%; 0.2684 g) were mixed in the molar ratio 85:15, ground evenly in an agate mortar, and put into a silicon glass tube, which was vacuumsealed using a home-made sealing machine.The resulting ampoule was placed in a furnace (SG-XQL1200) and heated up to 473 K for 5 min with a heating rate of 10 K min À 1 , and then heated up to 1373 K for 2 h 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 singlecrystal X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details of AlFe 0.95 are summarized in Table 1, while crystal data, data collection and structure refinement details of the AlFe 0.82 and AlFe 0.84 phases are summarized in Table S3 of the supporting information.Different options for refinement are listed in Table S2.For the AlFe 0.95 phase, the maximum and minimum residual electron densities in the final difference map are located 1.30A ˚and 0.72 A ˚from Al1.Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2016/6 (Sheldrick, 2015b), DIAMOND (Brandenburg & Putz, 2017), Mercury (Macrae et al., 2020) and publCIF (Westrip, 2010).
Figure 1The AlFe 0.95 structure (one unit cell), with displacement ellipsoids at the 95% probability level.

Figure 4
Figure 4The variation of residual R 1 versus � for the title compound, obtained by refining the model with different values for �.The minimum of the curve is at � = 0.05.

Table 1
Experimental details.