The dissection of atomic clusters with local structure in Ga-Sn alloy melts Open Access

It is difficult to characterize the liquid structure because of the absence of long-range atomic order in the liquid state.¹ Numerous of effort have been devoted to understanding the structure of liquid metals and alloys since the accurate information of liquid structure is essential to analyze the microstructure and physicochemical properties of its solid.² In 1952, Frank proposed that icosahedral short-range order (ISRO) presents in liquids initially.³ Following his work, many researchers have found the icosahedral short-range order and distorted icosahedral short-range order in the bulk metallic melt.^4–10 In addition, some researchers also found that the ISRO is evidenced already above the melting point and becomes more pronounced in the undercooled state with decreasing temperature.^11,12

However, it is still an open discussion whether the icosahedral ordering can occur in all liquid metals or not. From all the literatures that have been reported, the local atomic structure can be depicted according to the coordination numbers and interatomic distances. The local point symmetry of the short range order in liquids still remains poorly understood in condensed matter science. Thanks to X-ray absorption fine structure spectrum (XAFS), its sensitivity to short-range order character helps researchers to investigate the local structure of the metals and alloys melt recently.^7,13,14

We have investigated the local structures of the normal and undercooled liquid Ga_85.8In_14.2 alloy by using X-ray absorption spectroscope and the results indicated that the 13-coordinated polyhedron gradually evolves into 12-coordinated polyhedron with decreasing temperature in our previous report.¹⁵ In this work, the structure of the liquid hypoeutectic and eutectic GaSn alloy will be measured firstly, followed by dissecting the atomic clusters with local structure in GaSn alloy melts and establishing the atomic clusters model. By this work, we found that the icosahedral short-range order is not the dominant atomic cluster in the liquid Ga-Sn alloy.

XRD and XAFS measurements were performed on the Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy liquid sample. The sample used in this work were prepared with gallium and tin of high purity (99.999%).

The XRD measurements were carried out using a diffractometer with Mo-Kα-radiation (wavelength λ= 0.7089 nm), the graphite monochromator and the tantalum heater. The sample was put onto an yttria crucible, with a size of 23 mm×18 mm×8 mm. The scanning voltage of the X–ray tube was 40 kV, the current was 30 mA, the exposure time was 20s, the measured 2θ angle was from 5° to 90° and the scanning step was 0.2°. The detailed data processing can be found in Ref. 16. The important parameters used in the study of liquids and amorphous materials are S(Q) and g(r). The S(Q) represents the structure of the alloy melt and is obtained by normalized scattering intensity directly measured from diffraction experiment data. The g(r) was defined as the probability of finding another atom at a distance r from an atom at the origin position (at the point r =0).¹⁷ The atomic g(r) is derived from the Fourier transformation of S(Q).

The Ga-K edge and Sn-K edge X-ray absorption spectra were recorded at the Beamline BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF) under room temperature. The electron storage ring was operated at 3.5 GeV and the maximum stored current was 300 mA. The data were collected in fluorescence mode by using a fixed-exit double-crystal Si (311) monochromator. The spot size at the sample location was 2 μm×2 μm. Harmonics were rejected by using a grazing incidence mirror. The data were collected in transmission mode using ion chambers with Ar/Kr filler gas. The calculations of the X-ray absorption near-edge structure (XANES) spectra, were performed using the ab-initio multiple-scattering FEFF8 program^18,19 in a real space which gives us an appropriate framework to study systems with lack of spatial symmetry.

Fig. 1 shows the results of the diffraction experiment. In the Fig. 1(a), we can see that an asymmetrical first peak on S(Q) curve and a ‘shoulder’ on the right side of the first peak. The similar phenomenon was also exhibited in the behavior of the high r–side of the first peak on g(r) of Ga_91.6Sn_8.4 alloy melt. The comparisons of the XANES spectra recorded at the Ga and Sn K-edge in the liquid Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy samples are shown in Fig. 2(a) and (b). As shown in Fig. 2(a), for the characteristics of Ga K-edge XANES spectra, the position and the slope of the absorption edge for the samples are quite similar. However, the intensities of the resonances depend strongly on the composition. In Fig. 2(b), for Sn element, the resonances intensities of different compositions are similar and the position and the slope of the absorption edge depend strongly on the composition.

The shoulder peak on the high Q side on the S(Q) curve has been found in the liquid Ga^20,21 and the shoulder usually represents the covalent bonds in the melt. Formerly, some researchers have found that a shoulder peak appears on the first peak of g(r) curve of the pure liquid tin near the melting point. And this kind of shoulder is also found on the g(r) curves of those liquid elements with covalent bonds in their solid state, e.g., silicon, carbon, and germanium.^22,23 The shoulder is attributed to the residual covalent bonds of solids turning into the liquids. Moreover, on the g(r) of liquid Ga_91.6Sn_8.4 alloy, a small hump (rather than a shoulder) appears between the first peak and the second one. According to the physical meaning of the hump on the g(r) curve, the g(r) of liquid Ga_91.6Sn_8.4 alloy demonstrate that some atoms move amid the nearest atom shell and the next one around the center atom, meaning the atoms of Ga outwards and those of Sn inwards.²⁴ 

The first peak height S(Q₁) indicates the intensity of the interaction between atoms.²⁵ The inset of Fig. 1(a) shows the S(Q₁) for liquid Ga₉₅Sn₅ alloy is larger than that for liquid Ga_91.6Sn_8.4 alloy at room temperature, indicating a more ordered melt structure. The peak height of g(r) indicates the atomic ordering in the alloy melt, and the peak position of g(r) indicates the average nearest neighbor distance between any two atoms. Larger value of the first peak height g(r₁) represents the higher ordering of atoms in the liquid alloy system.²⁶ The inset of Fig. 1(b) shows the g(r₁) for the liquid Ga₉₅Sn₅ alloy is higher than that for the liquid Ga_91.6Sn_8.4 alloy which indicating that the melt is more ordered. Based on the g(r), the mean nearest neighbor distance (r₁) of liquid Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy was 2.81 Å and 2.85 Å, respectively. The r₁ is bigger for the intermetallic composition with more Sn content. Aiming to dissect the local structure in the liquid Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy, we have performed a detailed XANES study at the Ga and Sn K-edge.

XANES corresponds to the first 30-50 eV of XAFS from the absorption edge and it is related to the excitation process of a core electron to bound and quasi-bound states.²⁷ The XANES is very sensitive to the changes in the chemical environment around a selected element and provides information about the oxidation state (valence) and coordination symmetry of the absorbing atom.¹⁹ So that the changes in the XANES spectra are associated with the local coordination and the density of unoccupied electronic states.²⁸ The characteristic of the Ga and Sn K-edge XANES spectra of the liquid Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy indicates that Ga atom in liquid Ga₉₅Sn₅ alloy has the same electron configuration and different site symmetry with Ga_91.6Sn_8.4 alloy. So that, the electron configuration of the Sn atoms in Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy is different but the site symmetry of its coordination atoms was similar.

The Ga and Sn K-edge XANES spectra (normalized to an edge step of 1) for the Ga_91.6Sn_8.4 alloy melt at room temperature and the simulated XANES spectra are shown in Fig. 3. Based on the analyzing results of XRD measurements and dissecting the clusters’ structures, estimated parameters of the cluster models could be predicted. In order to get information about the local atomic order in the Ga_91.6Sn_8.4 alloy melt, the models were refined iteratively to obtain the best agreement with the experimental results. The cluster models used for simulating the local atomic arrangement which can fit the XANES spectra are also shown in Fig. 3. All of them are 12-coordinated polyhedrons which consist of one center atom and twelve coordination atoms distributed on the first shell, but they are not the common icosahedral structure.

For the Ga atom and Sn atom clusters in the liquid Ga_91.6Sn_8.4 alloy, there are both two Sn atoms and ten Ga atoms around the center atom. In the center Ga atom cluster, the mean atom distances from Ga and Sn to the center Ga atom are 2.742 Å and 3.230 Å, respectively. And the two neighbor Sn atoms distribute on either side of the center Ga atom. However, in the center Sn atom cluster, the interaction between two neighbor Sn atoms is also strong and they distribute on one side together. The mean atom distances from Ga and Sn to the center Sn atom are 2.919 Å and 3.270 Å, respectively.

Fig. 4 shows the normalized and simulated Ga and Sn K-edge XANES spectra of the Ga₉₅Sn₅ alloy melt at room temperature. The atomic configuration in the cluster fitted the XANES spectra is also given in the picture. Similar to the local structure in Ga_91.6Sn_8.4 alloy, the atom cluster is 12-coordinated polyhedron but not the common icosahedral structure. The center Ga atom cluster consists of two Sn and ten Ga coordination atoms and the average distances between a center atom and Ga and Sn atoms in the first nearest-neighbor shell are 2.692 Å and 3.233 Å, respectively. Nevertheless, a center Sn atom cluster consists of one Sn and eleven Ga coordination atoms and the mean atom distances from Ga and Sn atoms in the first nearest-neighbor shell to the center Sn atom are 2.919 Å and 3.300 Å.

From the Fig. 3 and Fig. 4, we also can see that only the center Ga atom cluster in the Ga₉₅Sn₅ alloy can be divided into two roughly symmetrical structure, and the straight line connected the two Sn atoms was the symmetry axis. In addition, the most important difference between the liquid Ga_91.6Sn_8.4 and Ga₉₅Sn₅ alloy structure is the center Sn atom cluster, and the electron configuration of the center Sn atoms in Ga₉₅Sn₅ and Ga_91.6Sn_8.4 alloy is different.

In general, the cluster size in the liquid is represented by the mean nearest neighbor distance r of the center atom. The r of the center atom can be defined as

and

for A and B species, respectively. Where r is mean distance and n is coordination number of the current apecies; the subscripts A and B denote the species A and B. According to the equation (1), the r_Ga and r_Sn of lquid Ga_91.6Sn_8.4 and Ga₉₅Sn₅ alloy are calculated respectively. In lquid Ga_91.6Sn_8.4 alloy, the r_Ga is 2.823 Å and the r_Sn is 2.9775 Å. The r_Ga is 2.782 Å and the r_Sn is 2.951 Å in lquid Ga₉₅Sn₅ alloy. In addition, the mean nearest neighbor distance r of the Ga_xSn_100-x alloy can be calculated by

Therefore, the r of the Ga_91.6Sn_8.4 and Ga₉₅Sn₅ are 2.836 Å and 2.791 Å. It can be observed that its value is approximately equal to the r₁ obtained from the XRD results.

In conclusion, the cluster structure was dissected by the atomic local structure in the hypoeutectic and eutectic Ga-Sn alloy melt. The center Sn atom clusters of Ga₉₅Sn₅ and Ga_91.6Sn_8.4 with different atomic configurations, Sn atoms aggregate in Ga_91.6Sn_8.4 alloy, giving rise to a phase separation that exhibits compositional disorder. It is found that the normal icosahedral short-range order was not the dominant atomic cluster in the liquid Ga-Sn alloy.

This research was supported by the National Natural Science Foundation of China (Grant Nos. 51371107, 21777065) and the Natural Science Foundation of Shandong province (Grant No. ZR2017PEM002).