I. INTRODUCTION
Al–Cu alloys are widely used in aerospace, transportation, and other fields due to their good comprehensive mechanical properties (Cheng et al., Reference Cheng, Chen, Tao, Liu, Yin and Weng2022). And since the properties of materials are closely related to their crystal structure, it is necessary to study the crystal structure of Al–Cu alloy compounds. Al4Cu9 is an intermetallic compound in the Al–Cu binary system, it crystallizes in a cubic crystal structure, and its space group and Pearson symbol are 
$P\bar{4}3m$ and cP52, respectively (Westman, Reference Westman1965). In recent decades, few investigations have been conducted on intermetallic compound with the Al4Cu9 structure type. In addition, these studies have focused mainly on compounds of binary systems. For instance, binary compounds Cu7.8Ga5.2, Cu8.45Ga4.55, and Cu8.06Ga4.94 were reported in a study of structural changes during the solidification of copper–gallium alloys by Tikhomirova et al. (Reference Tikhomirova, Pikunov, Marchukova, Tochenova and Izotova1969). The structure of Cu9Ga4 compound was refined from single-crystal diffractometer data by Stokhuyzen et al. (Reference Stokhuyzen, Brandon, Chieh and Pearson1974). A high-temperature phase Cu9In4 was synthesized by Che and Ellner (Reference Che and Ellner1992), who reported the powder crystal data. The crystal structure data of these binary compounds are included in the ICSD database (2015). As far as we know, only few studies and reports on Al–Cu–Ga ternary intermetallic compounds. According to the ICSD database, simultaneously review the Powder Diffraction File (PDF) of the current International Center for Diffraction Data (ICDD), no ternary intermetallic compounds of Al–Cu–Ga system have been reported in these databases, but an intermetallic compound Al2Cu9In2 was discovered in a similar system Al–Cu–In, and has Al4Cu9 (Westman, Reference Westman1965) crystal structure.
In this paper, a new ternary intermetallic compound Al3GaCu9 in the Al–Cu–Ga system was synthesized experimentally, and the results of crystal structure refinement, powder diffraction data, and experimental value of the reference intensity ratio (RIR) of the intermetallic compound Al3GaCu9 are reported.
II. EXPERIMENTAL
A. Alloy sample preparation of Al3GaCu9
The alloy ingot of Al3GaCu9 intermetallic compound was prepared by arc melting with the total mass was 1.5 g. The initial materials such as Al, Cu, and Ga elements in the alloy ingot were small pieces and the purity was more than 3 N. To obtain a uniform composition, the alloy ingot was remelted many times, and the melting losses were less than 0.5%. After melting, the alloy ingot was annealed in a high-temperature furnace at 823 K for 720 h, and cooled to room temperature in the furnace.
B. Powder diffraction patterns collection
Alloy ingot of Al3GaCu9 was ground in an agate mortar and pestle to particle sizes of less than 30 μm. The powder X-ray diffraction (PXRD) patterns of the Al3GaCu9 intermetallic compound were collected at ambient temperature by a powder X-ray diffractometer (Smart Lab 9 kW, Rigaku Corporation). The X-ray goniometer optics had a goniometer radius of 300 mm and was equipped with an X-ray source of Cu K(alpha) radiation (lambda −1.54060 Å) and a graphite monochromator. The step scanning collection method was adopted, and the collection conditions were as follows: step size 0.02°, a count time 2 s per step, two-theta scan range 10° to 130°, operating voltage 40 kV, and current 150 mA. To reduce the displacement systematic error, high-purity silicon was added as the internal standard during the testing. Finally, 50 wt.% Al2O3 was added to the Al3GaCu9 compound; the PXRD pattern was collected and used to obtain the experimental value of RIR (Walter and Schreiner, Reference Walter and Schreiner1995).
III. RESULTS AND DISCUSSION
After collecting the PXRD patterns of the Al3GaCu9 intermetallic compound, the program JADE 6.5 (Materials Data Inc., 2002) was used for indexing. The results show that the compound crystallized in a cubic crystal structure. Comparing the crystallographic characteristics of Al3GaCu9 with the structural types in the database revealed that the Al3GaCu9 intermetallic compound has an Al4Cu9 structure type, where the space group and Pearson symbol are 
$P\bar{4}3m$ (No. 215) and cP52, respectively. The Al4Cu9 structure type has eight atomic positions of the elements, the occupancy of elements in all positions is 1, the number of atoms and formula units per unit cell are 52 and 4, respectively. Therefore, these parameters of Al4Cu9 structure and the indexed lattice parameter of Al3GaCu9 were used as the initial parameters for Rietveld refinement using the DBWS9807a program (Young et al., Reference Young, Larson and Paiva-Santos2000).
To obtain the lattice parameter as accurately as possible and reduce the systematic error, a high-purity silicon internal standard for two-theta correction was used. After correction, the lattice parameter was found to a = 8.7134(2) Å. The PXRD data for Al3GaCu9 are listed in Table I. During the refinement, the pseudo-Voigt function was used to simulate the peak shapes. The peak shape fitting of the PXRD patterns for Al3GaCu9 is shown in Figure 1. After the refinement of 32 parameters, including the lattice parameter, atomic positions, thermal parameters, and occupancy and mix parameters, the refinement results were shown via the DMPLOT program (Marciniak and Diduszko, Reference Marciniak and Diduszko1997) and are summarized in Table II. As listed in Table II, lattice parameter a = 8.7132(3) Å, unit-cell volume V = 661.52 Å3, and calculated density D calc = 7.26 g/cm3. The residual factors converge to R p = 2.96%, R wp = 4.06%, R exp = 2.57%, and S = 1.57. These results combined with the results of peak shape fitting, clearly show that the refinement is reliable. Table III summarizes the final results of atomic positional parameters of Al3GaCu9. As presented in Table III, Ga atom replaces the 4e position of Al in Al4Cu9 structure and the occupancy is 1, whereas the other atoms remain unchanged. Table IV lists the bond lengths (<3.0 Ǻ) and nearest neighbors of each atom for Al3GaCu9. The number of nearest neighbor atoms of Ga(4e), Al(12i), Cu1(4e), Cu2(4e), Cu3(4e), Cu4(6f), Cu5(6g), and Cu6(12i) are 10, 11, 12, 12, 13, 13, 13, and 11, respectively. The structure, a-axis projection, and the coordination environment of some atoms in Al3GaCu9 are shown in Figure 2(a)–(d), respectively. Finally, The RIR experimental value of 7.04 was obtained by analyzing the strongest line ratio of Al3GaCu9 and corundum when in a 50:50 mixture by weight (Hubbard et al., Reference Hubbard, Evans and Smith1976; Snyder, Reference Snyder1992).
Figure 1. Peak shape fitting of the PXRD patterns of Al3GaCu9. “+” and “|” symbols represent the observed patterns and the possible positions of Bragg reflections, respectively. The solid line represents the calculated patterns, and the bottom curve represents the residual between the observed and calculated patterns.
Figure 2. Structure, a-axis projection, and the coordination environment of some atoms for Al3GaCu9. (a) Al3GaCu9 structure, (b) Al3GaCu9 a-axis projection, (c) Ga atom, and (d) Al atom.
TABLE I. Powder X-ray diffraction (PXRD) data of Al3GaCu9.
a Δ2θ = 2θ obs–2θ cal.
TABLE II. Rietveld refinement results of Al3GaCu9.
TABLE III. Atomic positional parameters of Al3GaCu9.
TABLE IV. The bond lengths (<3.0 Å) and nearest neighbors of each atom for Al3GaCu9.
IV. CONCLUSION
The new ternary intermetallic compound Al3GaCu9 was successfully synthesized. The crystal structure was determined by the Rietveld refinement method based on the PXRD pattern, The results show that the compound has a cubic cell with the Al4Cu9 structure type (space group 
$P\bar{4}3m$ and Pearson symbol cP52). The lattice parameter a = 8.7132(3) Å, unit-cell volume V = 661.52 Å3, calculated density D calc = 7.26 g/cm3, and Z = 4. The residual factors converge to R p = 2.96%, R wp = 4.06%, and R exp = 2.57%. The experimentally obtained RIR value is 7.04.
V. DEPOSITED DATA
CIF and/or RAW data files were deposited with ICDD. You may request this data from ICDD at pdj@icdd.com.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 51861001), Young and Middle-Aged Teachers Basic Research Ability Improvement Project of Guangxi Universities (2022KY1096), and Baise Scientific Research Foundation (No. 20221465, 20221466).
CONFLICT OF INTEREST
The author has no conflicts of interest to declare.
