ISOTHERMAL SECTION OF THE Ni–Mn–Sb TERNARY SYSTEM AT 773K

The isothermal section of the Ni-Mn-Sb ternary system at 773 K was measured by means of 117 alloys which were analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and electron probe microanalysis (EPMA) techniques. The existence of 7 binary compounds, namely NiMn, Mn2Sb, MnSb, NiSb2, NiSb, Ni5Sb2, Ni3Sb and 2 ternary compounds, namely Ni2MnSb and NiMnSb were confirmed for this isothermal section. The four binary compounds Ni3Sb (Cu3Ti structure, Pmmn space group), Ni5Sb2 (Ni5Sb2-type structure, C2 space group), NiSb2 (FeS2-type structure, Pnnm space group) and Mn2Sb (Cu2Sb-type structure, P4/nmm space group) in the binary systems Ni-Sb and Mn-Sb were stoichiometric compounds, the homogeneity ranges of which were negligible. However the five single phases in the Ni-Mn system and the two binary compounds MnSb and NiSb showed more or less homogeneity ranges formed by substitution of Mn and Sb for Ni atom. The Heusler compound μ (Ni2MnSb) has L21-type ordered structure with space group Fm-3m, a = 0.6017 nm. And the crystal structure for the Half-Heusler compound κ (NiMnSb) is C1b-type (F-43m) with a = 0.5961 nm. The approximate homogeneity ranges of the two ternary compounds μ and κ at 773 K were investigated.


Introduction
The magnetocaloric effect (MCE) is an intrinsic property of magnetic material, which has attracted much attention for its potential application in magnetic refrigeration. A large MCE usually occurs as the result of first-order magnetic transition in materials, such as Gd-Si-Ge [1], La-Fe-Si [2], and Fe-Mn-P-As [3,4]. To date, the large inverse MCE have been reported in Ni-Mn based Heulser alloys, such as Ni-Mn-Sn [5], Ni-Mn-In [6,7], and Ni-Mn-Sb [8], which undergoes a first order structure transition (martensitic transition) from a high-symmetry austenitic phase to a low-symmetry martensitic phase with an abrupt drop of magnetization. The large MCE has been studied in compounds doping with Si, Ga, Ge and Co in Sb site in Ni-Mn-Sb system [9][10][11][12][13]. Understanding the phase relationships of Ni-Mn-Sb system is helpful for the development of relative materials. Up to now, no phase diagram containing the whole range of the Ni-Mn-Sb system has been constructed. In this work, we investigated the phase equilibria in the Ni-Mn-Sb ternary system at 773 K.
For the Mn-Sb binary system [18], there were two compounds -MnSb, and Mn 2 Sb with some solid solution ranges at 773 K. The Ni-Sb binary system was reported by Cha [19] in 1990 and lately revised by Zhang [20] in 2008. The main difference between them was that the chemical formulation for the compounds in the region of 20-30 at.% Sb was different. In Ref. [19], there were five intermetallics, and the nominal compositions were Ni 3 Sb, Ni 5 Sb 2 (HT), Ni 7 Sb 3 (LT), NiSb, and NiSb 2 . Zhang revised this binary system by combining lots of experimental information , namely the five intermetallics as δNi 3 Sb, βNi 3 Sb (HT), θNi 5 Sb 2 (LT), γNiSb and ζNiSb 2 , respectively [20]. According to Zhang's assessment, four intermetallics, except for the βNi 3 Sb (HT) phase, existed at 773 K in Ni-Sb binary system. Two ternary compounds Ni 2 MnSb and NiMnSb in the Ni-Mn-Sb ternary system have been reported [22,23]. Crystallographic data for all the intermetallic compounds related to this paper were taken from Ref. [24] and showed in Table 1.

Experimental
All the Ni-Mn-Sb alloys were prepared by melting in pure argon atmosphere, using a water-cooled copper tray and a non-consumable W electrode. The Mn, Ni, and Sb metals were used as raw materials, the purity is higher than 99.9 %. Ti was used as an oxygen getter during the preparing process. The samples were melted three times to ensure complete melt and homogeneous composition. 117 alloy ingots were prepared. As Sb and Mn evaporate, additional 2 -3% Sb and Mn were added to compensate losses during arc-melting, especially at high Sb and Mn content. The weight loss of the ingots was less than 1 wt.%. The melted ingots were sealed in vacuum quartz tubes and then put in a muffle furnace for homogenizing heating at different temperatures to reach fine homogenization. The heat-treating temperature for the alloys was selected by the phase diagrams of the Mn-Ni, Ni-Sb and Mn-Sb systems and the differential thermal analysis (DTA) result for some key alloys. The homogenization heating was performed at 873 K for 20 days for the Sb-rich alloys  [17], Mn-Sb [18] and Ni-Sb [20] with more than 40 at.% Sb, and 1173 K for 15 days for the the rest of the specimens. Then all of the ingots were cooled to 773 K at the rate of 10 K/min, and held at 773 K for 30 days. All ingots were quenched in liquid nitrogen after heat-treating process. Most ingots were cut into two parts for XRD analysis and morphology observation. The alloys were ground in an agate mortar into micrometer powder for X-ray diffraction. The phases in each sample were confirmed through XRD analysis using a Bruker diffractometer, the type of D8 Advance SS/18kW and with Cu-Kα radiation at 40 kV, 200 mA. The data were obtained in the range of 2θ from 20 o to 80 o at a step length of 0.02 o . The phase analysis and structure refinement were carried out using the JADE 6.0 and Topas 3.0 softwares. After standard metallographic specimen preparation, the micromorphology and the compositions of each phase were measured by EPMA (JXA-8530F, JEOL, Japan). Pure elements Mn, Ni and Sb were used as standards and the EPMA measurements were performed at 20.0 kV. The microstructures and energy dispersive (EDS) spectroscopy analyses were carried out using a Hitachi S-4700N VP-SEM scanning electron microscopy (SEM).

Phase analysis and identification
Phase identification was performed based on the Rietveld refinement results and equilibrium phase composition determined by EPMA and EDS techniques. In this work the XRD analysis of the ingots in the boundary binary systems confirmed the existence of seven binary compounds, βNiMn in the Ni-Mn system, Mn 2 Sb and MnSb in the Mn-Sb system, NiSb 2 , NiSb, Ni 5 Sb 2 and Ni 3 Sb in the Ni-Sb system at 773 K, in a fine agreement with those reported in this system [17][18][19][20]. Two ternary compounds Ni 2 MnSb and NiMnSb existed in the 773 K isothermal section in this system. The PDF files for the compounds above-mentioned can be found on JCPDS PDF cards (2004).
Three representative XRD patterns for the alloys on the Ni-Mn and Mn-Sb boundary were presented in Fig.2. It can be seen that the three alloys Mn 0.6 Ni 0.4 , Mn 0.8 Sb 0.2 , Mn 0.25 Sb 0.75 were located in the two-phase regions of αNi + βNiMn, αMn + Mn 2 Sb and MnSb + εSb, respectively, from which, the three binary compounds βNiMn, Mn 2 Sb and MnSb were confirmed.
For the Ni-Sb binary system, 4 binary compounds NiSb 2 , NiSb, Ni 5 Sb 2 and Ni 3 Sb were confirmed at 773 K. In earlier time, P. Nash found a low-temperature phase θNi 7 Sb 3 [19] in Ni-Sb system, but the later research of Y.B. Zhang [20] and C.H. Li [25] showed that this low-temperature phase's real equilibrium composition should be Ni 5 Sb 2 . The Ni 5 Sb 2 phase was confirmed in this work, which was well agreed with the later research results.
Several representative Rietveld structure refinement results for the compounds on Ni-Sb binary system were shown in Fig.3. According to the patterns above, we can see that the Ni 0.12 Sb 0.88 , Ni 0.45 Sb 0.55 , Ni 0.72 Sb 0.28 , and Ni 0.9 Sb 0.1 alloys were located in the phase regions of Sb+NiSb 2 , NiSb+NiSb 2 , Ni 5 Sb 2 and γNi+Ni 3 Sb, respectively, from which, the four binary compounds NiSb 2 , NiSb, Ni 5 Sb 2 and Ni 3 Sb were confirmed in the Ni-Sb binary boundary at 773 K.
Two ternary compounds, Ni 2 MnSb and NiMnSb, were verified in the Ni-Mn-Sb ternary system at 773 K. The Heusler alloy Ni 2 MnSb has L2 1 -type totally ordered structure [22] (S.G. Fm-3m, a = 0.6011 nm), in which, Ni atoms located at all the apex angle positions of the 8 little cubes while 4 Mn atoms and 4 Sb atoms occupied the 8 cubic center positions alternately and orderly ( Fig.4(a)). The Half-Heusler alloy NiMnSb has C1 b -type ordered structure [23] (S.G. F-43m, a = 0.5932 nm), in which, half of the Ni atoms which were located in the middle of the large    (Fig.4(b)), contrasting with the compound Ni 2 MnSb were missing.
The XRD patterns of the compounds Ni 2 MnSb and NiMnSb were shown in Fig.5. The biggest difference between them was the relative diffraction peak intensity of the first and second peak. In Ni 2 MnSb phase the intensity of the (111) lattice plane was almost twice of the (200) plane, while in NiMnSb phase, they were nearly the same. The space group and structure type for Ni 2 MnSb was S.G.Fm-3m, L2 1type. And those for NiMnSb was S.G.F-43m, C1 btype. Table.2 showed the Rietveld refinement data result of their XRD patterns. From the tab.2 we can see that the Rietveld refinement factors were so small and the Rietveld result was reliable.

Determination of the phase regions and homogeneity ranges
By comparing the XRD patterns analysis and the EPMA or EDS compositional results, the phase compositions in each alloy were identified and the phase homogeneity regions in this system were determined. The XRD patterns of typical alloys in this isothermal section were shown in Fig.6. We can see that the two alloys of sample 6#-Ni 0.15 Mn 0.15 Sb 0.7 and 47#-Ni 0.1 Mn 0.7 Sb 0.2 (labeled with A and B in Fig.9) were located in the ternary regions of Sb + MnSb + NiSb 2 , NiMnSb + Mn 2 Sb + αMn, respectively. Fig.7 showed the typical Rietveld structure refinement results and the SE (Secondary electron) images for 59# -Ni 0.3 Mn 0.3 Sb 0.4 and 60# -Ni 0.2 Mn 0.4 Sb 0.4 (labeled with capital letter C and D in Fig.9) compounds, from which we can see that this two alloys were located in the phase regions of NiMnSb + NiSb + MnSb and NiMnSb + MnSb, and the XRD results were in good agreement with the EDS results.  Fig.9), from which we can see that the four alloys were located in the phase region of Ni 2 MnSb + γNi, Ni 2 MnSb + NiSb, NiMnSb + NiSb, and NiMnSb + βMn, respectively. The EPMA images, presented in Fig. 8(b), indicated that the dark region was recognized as γNi phase and the grey region as Ni 2 MnSb phase. The EPMA images in Fig. 8(d) showed that the major phase was Ni 2 MnSb (dark matrix) and the minor phase was NiSb (white slice). In the same way, it was verified that the dark region was NiMnSb and the grey region was NiSb in Fig.  8(f), while the dark matrix was βMn and the grey region was NiMnSb in Fig. 8(h). The XRD analysis was in good agreement with the EPMA results. From Figs. 8b and 8d, as well as Figs. 8f and 8h, we can see that the contrast of the Ni 2 MnSb and NiMnSb phases is drastically changing. This is because the signal intensity in EPMA picture is proportional with the average atomic number Z. The average Z values of