The effect of impurity kinds and content on phase constituent and the microstructure of LaSi alloy

LaSi alloy with 1:1 atomic ratio was prepared by vacuum arc melting process, and different content impurities of Fe, Ni and Cu were added into the LaSi alloy. The phase constituent and microstructure of LaSi alloy with different impurities were studied using XRD and SEM, respectively. Experimental results showed that the LaSi alloy exhibited a new phase LaSi2-x when the added Fe content reached more than 300ppm. And the main diffraction peak of LaSi phase was shifted to a lower angle with the increase of Fe content. Similar to the effect of Fe impurities, the phase LaSi2-x appeared when the Ni impurity was added into LaSi alloy, while the diffraction intensity of main peak reduced by about 5 times compared to high-pure LaSi alloy. There was no LaSi2-x phase when the Cu was added into LaSi alloy, and the diffraction intensity of the main peak of LaSi increased by 2 times compared to the high-purity LaSi alloy. The microstructure of LaSi with Fe and Ni impurities displayed dark grey network. The LaSi alloy exhibited almost single-phase microstructure when the Cu impurity content reached 1526ppm. The Fe and Ni impurities exerted an obvious effect on phase and microstructure of LaSi alloy, while the Cu impurities played little effect on the phase and microstructure of LaSi alloy.


Introduction
Rare Earth metals can react with silicon (Si) to form rare-Earth metal silicide, which broadens the material systems and material synthesis methods of experimental research [1][2][3][4]. Rare Earth metal silicide is difficult to form because of the complex electronic structure of rare Earth metals and the very active chemical properties. In addition, the factors affected by impurities are serious. In recent years, La, Ce, Nd, Er, Y and Gd silicides in rare Earth silicides have been studied [5][6][7][8].
Rare-Earth metal silicide can form good ohmic contact on the surface of N-type Si. And a high potential barrier can be formed on the surface of P-type Si, which has special fluorescence and electroluminescence. The luminous effect has been applied to related electronic devices [9][10][11]. Many preparation methods of rare Earth silicides have been reported, including the direct reduction reaction of rare Earth metal oxides with Si, arc melting of rare Earth metals and Si powder, ion implantation, deposition annealing. Impurities are easy to be introduced into the sample during the preparation process, which will affect the quality of the sample.
Lanthanum silicon (LaSi) as basic materials are ideal in devices such as substrates of the integrated circuits, host lattices for light emitting phosphors, ohmic contacts and rectifying contacts [12][13][14]. Because the electronic structures of La and Si are complex and their chemical properties are very active, the factors affected by impurities are serious. The purity of rare-Earth metal has a serious impact on electrical properties of contacts and electroluminescence properties [15][16][17][18][19]. In fact, some impurities in LaSi have a serious impact, while some Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
impurities will not affect the performance of downstream products of LaSi. Therefore, it is of great significance to analyze the influence of impurities on LaSi and related products.
In this paper, the LaSi alloy was prepared by vacuum arc melting process, and the impurity kinds and content of LaSi alloy were studied. In the preparation processes of metal La, different metal impurities are easy to be introduced into La, such as Ni and Cu coming from fluorination and melting process. Besides, some metal impurities can be introduced into LaSi alloy in the processes of arc melting process. In general, it is not easy to remove all the impurities out of La and LaSi alloy. Therefore, it is very necessary to target a constituent removing of certain impurities out of metal La and strictly control corresponding processing processes. Therefore, the effect of Fe, Ni and Cu impurity with different content by arc melting process on phase constituent and the microstructure of LaSi alloy were analyzed and discussed, which can provide guidance for the purification of La and the preparation of LaSi alloy.

Experiment
The raw materials were lanthanum lumps and silicon lumps, both with purities higher than 99.9%. The materials were weighed in the atomic percentage of LaSi. Then, the LaSi alloy was prepared by arc-melting under an Ar atmosphere in a water-cooled copper tray. The Fe powders were introduced into four LaSi samples. And the Ni powders and Cu powders were introduced into four LaSi cylindrical ingots, respectively. In the current study, four different content impurity Fe-doped LaSi samples were melted three times by electric arc furnace. And the Ni-doped and Cu-doped LaSi samples were fabricated with the same processing technology. Chemical analysis of the LaSi and doped LaSi alloys was performed using inductively coupled plasma emission optical spectroscopy (ICP-EOS). The chemical composition of the alloys after melting is shown in table 1.
The ten samples of high-purity LaSi cylindrical ingot were prepared and the schematic diagram of samples in different areas is shown in figure 1. The phase constituents of the ten samples with different impurities were identified by x-ray diffraction (XRD, D/max2500Pc) at a scanning speed of 4°min −1 , ranging from 20°to 70°with Co Kα radiation in the air at room temperature. The microstructure and chemical compositions of the samples were investigated using a scanning electron microscope (SEM), equipped with an energy-dispersive spectrometer (EDS). The phosphors were prepared by the same process for LaSi alloys with different kinds of impurity contents.

Results and discussion
3.1. LaSi alloy X-ray diffraction patterns of the LaSi alloy cylindrical ingot in different areas are shown in figure 2. It can be seen that the ingots at different positions prepared by arc melting process with atomic ratio of La and Si of 1:1 were mainly composed of LaSi phase. This result was highly consistent with the La-Si phase diagram reported in the previous study [20]. In addition, the characteristic peak position and strength of the main phases of LaSi alloy at different positions of ingot were basically similar. Besides, there were no other hetero-phases appearing. In general, the alloy exhibited a completely single LaSi phase and it was very homogeneous, which proved that the arc-melting process was capable of producing high-pure LaSi alloy.  Figure 3 shows the x-ray diffraction patterns and the local enlarged patterns of the LaSi alloy with different Fe impurities content. It can be seen that the new phase LaSi 2-x appeared at diffraction angle of 37.8°-38.3°and the diffraction peak at 43°-45°corresponding to La 5 Si 4 was also observed. The main peak of LaSi was also changed significantly after the addition of Fe impurities and it shifted to a low angle with the addition of Fe impurities as shown in the local enlarged view. In addition, the peak intensity of (020) crystal plane increased significantly with the addition of Fe impurity content. These phenomena indicated that the lattice expansion became more and more serious with the addition of Fe impurities. It is reasonable to deduce that lattice expansion occurred because some Fe impurities were dissolved in LaSi alloy. Figure 4 shows the LaSi phase diagram. As can be seen, the increase of local La content resulted in the formation of La 5 Si 4 phase. Furthermore, the increase of local Si content resulted in the formation of LaSi 2-x . The LaSi 2-x was a phase with certain vacancy defects which were obtained by arc-melting Fe and LaSi. Figure 5 shows the x-ray diffraction and the local enlarged patterns of the LaSi alloy with different Ni impurities content. It was apparent that the LaSi 2-x peak appeared at diffraction angle of 37.8°-38.3°and La 5 Si 4 appeared at diffraction angle of 43°-45°. The diffraction intensity of LaSi 2-x increased gradually with the addition of Ni. The transformations of phase type were almost the same as LaSi alloy with Fe impurities. It was worth noting that the LaSi phase exhibited a strong (111) and (102) texture of LaSi. The (111) peak shifted to a high angle with the addition of Ni impurities, but the angle of (102) peak remained unchanged from the local enlarged view. This result was different from the effect of Fe on the phase constituent of LaSi. There was no obvious lattice expansion of LaSi because the radius of Ni atom (R Ni = 0.1246 nm) was smaller than that of Fe  atom (R Fe = 0.126 nm). The LaSi alloy with different Ni showed lower diffraction intensity of main peak than the high-pure LaSi alloy, and it was even reduced by about 5 times, which can be attributed to the fact that the proportion and the distribution area of LaSi 2-x and La 5 Si 4 phases increased. Therefore, it can be concluded that the LaSi phase was seriously affected by Ni impurity. The x-ray diffraction and the local enlarged patterns of the LaSi alloy with different Cu impurities content are shown in figure 6. It can be seen that no other miscellaneous peaks and LaSi 2-x phase appeared with the addition of Cu. However, the main peak of LaSi shifted to high angle with the addition of Cu content from the local enlarged view. It can be concluded that the addition of Cu impurities resulted in lattice contraction of the LaSi alloy. It was obviously noticed that the diffraction intensity of the main peaks of LaSi increased by 2 times with the addition of Cu impurities. The sharpness and high intensity of the diffraction peaks indicated the higher crystallinity was acquired by introducing Cu into LaSi alloy. The results revealed that influence of Cu impurities on the phase constituent of LaSi was different from that of Fe and Ni impurities. The solid solution mode of Cu was different from that of Fe and Ni. The radius atom of Cu (R Cu = 0.1276 nm) was larger than that of Fe atom (R Fe = 0.126 nm) and Ni (R Fe = 0.1246 nm). These features indicated that the properties of LaSi alloy with Cu will be better than with Fe and Ni.   Figure 7 shows the microstructure morphology of LaSi and LaSi with different Fe impurities. It can be seen that a completely single-phase microstructure of the as-cast alloys with 50 at% La and 50 at% Si as shown in figure 7(a). As for LaSi-Fe-321ppm shown in figure 7(b), dark grey angle block new phase appeared after doped Fe impurities, and the new phase could present coarse network microstructure. According to the EDS analysis in table 2, the addition of Fe made the atomic ratio of Si greater than that of La. Therefore, the network microstructure of angle block was Si-rich phase, and a few Fe impurities were enriched in the new phases. According to the XRD result, the Si-rich phase was LaSi 2-x . With the increase of Fe impurities content, the LaSi 2-x phase increased obviously as shown in figures 7(c)-(e). And it was unevenly distributed on the LaSi matrix. With the increase of Fe impurities content, a small amount of solid soluble Fe impurities content could be detected in LaSi matrix as shown in table 2. Figure 8 shows the microstructure morphology of LaSi and LaSi with different Ni impurities, respectively. It is noticed from figure 8(a) that a completely single-phase microstructure of LaSi can be seen. In addition, fine     Figure 9 shows the microstructure morphology of LaSi and LaSi with different Cu impurities, respectively. Rarely new phases appeared after adding Cu impurities. Only a few small gray phases can be observed when the Cu impurity content reached 1526ppm. It can be seen that Cu impurities exhibited relative enrichment according to the EDS result shown in table 4. According to the XRD pattern, the LaSi 2-x phase was not detected. There was almost single-phase microstructure of the as-cast LaSi alloy with Cu impurities. It can be concluded that the addition of Cu impurities exerted little effect on the phase and microstructure.

Conclusion
LaSi alloy with 1:1 atomic ratio was prepared by vacuum arc melting process, and different content impurities of Fe, Ni and Cu were added into the LaSi alloy, aiming at investigating the effect of different impurities on the phase constituent and microstructure of LaSi alloy. The main conclusions can be drawn as follows: (1) The addition of Fe impurities resulted in the appearance of new phases LaSi 2-x and La 5 Si 4 , and it resulted in the lattice expansion of LaSi alloy. The dark grey network microstructure and the angle blocks microstructure were formed and they were randomly distributed in LaSi matrix when Fe impurities were added. A few Fe was enriched in LaSi 2-x phase. The Fe impurities had adverse effects on phase constituent and microstructure.
(2) The small addition of the Ni impurities resulted in a significant phase transformation. Not only the new phase LaSi 2-x and La 5 Si 4 appeared, but also the diffraction intensity of the main peak of LaSi decreased by 5 times. The LaSi 2-x presented fine dark gray blocks which uniformly distributed on the LaSi matrix. Ni impurities had a serious effect on the phase constituent and microstructure.