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The phase relations in the Gd–Fe–Ga ternary system at 500 °C
Journal of Alloys and Compounds 479 (2009) 134–139
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The phase relations in the Gd–Fe–Ga ternary system at 500 ◦ C D.C. Liu, J.Q. Li ∗ , M. Ouyang, F.S. Liu, W.Q. Ao College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 10 December 2008 Accepted 15 December 2008 Available online 25 December 2008 Keywords: Rare earth alloys and compounds Phase diagram X-ray diffraction
a b s t r a c t The isothermal section (500 ◦ C) of the phase diagram of the Gd–Fe–Ga ternary system was investigated by X-ray powder diffraction analysis. Eleven binary compounds, GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , Fe3 Ga, Fe6 Ga5 , Fe3 Ga4 and FeGa3 , have been confirmed. Two ternary compounds, GdFe5.3 Ga6.7 and GdFe5 Ga7 , were found in this ternary system at 500 ◦ C. The compound GdFe5.3 Ga6.7 is orthorhombic ScFe6 Ga6 -type structure (space group Immm) with a = 0.8567 (9), b = 0.86960 (9) and c = 0.50782 (5) nm, while the compound GdFe5 Ga7 is tetragonal ThMn12 -type structure (space group I4/mmm) with a = 0.8651(1) and c = 0.50934 (6) nm. The isothermal section at 500 ◦ C consists of 16 single-phase regions, 31 two-phase regions and 16 three-phase regions. The maximum solid solubilities of Ga in GdFe2 , GdFe3 , Gd2 Fe17 are 9.2, 9.0, and 44.3 at.% respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in this compound is very small. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. Very limited solid solutions were measured in the other compounds. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental detail
Gallium has unique interaction with rare-earth (R)–transition metal (T) alloys. The studies of the phase diagrams of the Ho–Co–Ga, Sm–Co–Ga, Er–Fe–Ga and Sc–Fe–Ga ternary systems show rich ternary compounds or substitution derivatives in R–T–Ga systems [1]. The compound RFe12−x Gax which derives from the ThMn12 type structure, such as RFe6 Ga6 , is stable while RFe12 is not existed in the R–Fe binary system [2]. For the compound R2 Fe17−x Gax (R = Y, Sm Gd, Tb, Ho and Tm), the values of its Tc increase with Ga concentration strongly [3]. Systematic studies of the gallium substituted series R2 Co17−x Gax have shown that the phase relations and structural properties change strongly as a function of Ga content. As the gallium content increases, the hexagonal Th2 Ni17 -type structure transforms into the rhombohedral Th2 Zn17 -type structure, the Curie temperature and the saturation magnetization decrease [4]. From the reported phase diagram of the R–T–Ga ternary system, we found that the phases and phase relationships in theses system are different with different rare earth. A phase diagram plays an important role for materials researches. The investigation iron-rich corner of the Yb–Fe–Ga phase diagram was currently reported [5]. The investigation of the Gd–Fe–Ga has not been reported. In this work, we investigated the isothermal section of this ternary system at 500 ◦ C.
The polycrystalline Gd–Fe–Ga alloys were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in pure argon atmosphere. Gadolinium, Iron and Gallium with purity of 99.99% or higher were used as starting materials. Titanium was used as an O getter during the melting process. The alloys were re-melted at least three times in order to ensure complete fusion and homogeneous composition. The melted alloy buttons were sealed in evacuated quartz tubes containing titanium chips as an O getter placed in a resistance furnace for homogenization. The homogenization was performed at 800 ◦ C for 20 days for the alloys near Fe corner with Gd less than 33.3 at.% and Ga less than 50 at.%, and at 650 ◦ C for 25 days for the other alloys. They were cooled from their homogenization temperatures to 500 ◦ C, kept at 500 ◦ C for 3 days and then quenched in liquid nitrogen. About 130 samples in total were prepared for this investigation. X-ray powder diffraction (XRD) data was collected by a Bruker D8 Advance SS/18 kW diffractometer with Cu K␣ radiation. JADE 5.0 and Topas 3.0 softwares were used for phase analysis and structure refinement.
∗ Corresponding author. E-mail address: [email protected] (J.Q. Li). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.057
3. Results and discussion 3.1. Phase analysis The phase diagrams of the Gd–Fe, Gd–Ga and Fe–Ga binary systems were reported [6]. Four binary compounds, Gd2 Fe17 , Gd6 Fe23 , GdFe3 and GdFe2 exist in the Gd–Fe phase diagram, formed peritectically at 1335, 1280, 1155 and 1080 ◦ C, respectively. The compound Gd6 Fe23 was drawn with dot line, indicating this compound needs to be confirmed. In the work by Copeland et al. [7]. and Spedding et al. [8], only three intermetallic compounds were found, i.e. Fe17 Gd2 , Fe3 Gd and Fe2 Gd. Based on the Gd–Fe phase diagram reported in ref. [9], the compound Gd6 Fe23 formed peritectically at 1280 ◦ C but decomposes into Fe17 Gd2 and Fe3 Gd at about 1150 ◦ C.
D.C. Liu et al. / Journal of Alloys and Compounds 479 (2009) 134–139
135
Fig. 1. X-ray diffraction patterns for the compounds (a) Gd2 Fe17 and (b) Gd2 Fe10 Ga7 with Th2 Zn17 -type structure.
Five binary compounds, Gd5 Ga3 , Gd3 Ga2 , GdGa, GdGa2 and GdGa6 exist in the Gd–Ga phase diagram. The compound GdGa2 melts congruently at 1400 ◦ C, while the compounds Gd5 Ga3 , Gd3 Ga2 , GdGa and GdGa6 form peritectically at 1010, 1110, 1195 and 406 ◦ C, respectively. A large homogeneity range from 22 to 33.3 at.% Ga is shown in GdGa2 . Four intermediate phases exist in the Fe–Ga system: FeGa3 , Fe3 Ga4 , Fe6 Ga5 and Fe3 Ga, with polymorphic modifications for the last two: ␣-Fe3 Ga (cubic, Pm3m) for T < 619 ◦ C and -Fe3 Ga (hexagonal, P63 /mmc) for T > 619 ◦ C and ␣-Fe6 Ga5 (mono¯ clinic, C2/m) for T > 778 ◦ C and -Fe6 Ga5 (rhombohedral, R3m) for T < 778 ◦ C. All the binaries belonging to the Fe–Ga system except FeGa3 (tetragonal, P4/mnm), show a narrow range of homogeneity: ␣-Fe3 Ga (26–29 at.% Ga), ␣-Fe6 Ga5 (44.5–45.5 at.% Ga) and Fe3 Ga4 (57–58 at.% Ga). The structures, ScFe6 Ga6 -type (Immm) and ThMn12-type (I4/mmm), for the ternary compound GdFe6 Ga6 were reported in the Gd–Fe–Ga ternary system [10]. X-ray diffraction analysis for patterns of about 130 samples in this work confirmed the existence of the eleven binary compounds, GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , ␣-Fe3 Ga, ␣-Fe6 Ga5 , Fe3 Ga4 and FeGa3 , in the Gd–Fe–Ga ternary system at 500 ◦ C, which is in good agreement with those reported in ref. [6]. In this work and our previous work [11], we found the XRD pattern of the sample with composition of Gd6 Fe23 consists of those of Gd2 Fe17 and GdFe3 (see Fig. 1 in ref. [11]). We believed
Fig. 2. Variations of the lattice parameters a, c (a) and the unit cell volume V (b) of Gd2 Fe17−x Gax with Ga content (in at.% Ga).
that our result was in agreement with that reported in ref. [9]: the compound Gd6 Fe23 decomposes into Fe17 Gd2 and Fe3 Gd at higher temperature and should not existed at 500 ◦ C. Pure gallium is the only liquid phase at 500 ◦ C. A trace of the XRD pattern of GdGa6 appears in the XRD patterns of the alloys near Ga corner, but GdGa6 should not exist at 500 ◦ C since it decomposes at the temperature higher than 406 ◦ C. The alloys near Ga corner may contain the equilibrium liquid phase at 500 ◦ C. A small amount of GdGa6 phase may deposit from the liquid phase during the quenching. The crystal structures and lattice parameter data for the
Table 1 Crystal structures and lattice parameter data for the compounds in Gd–Fe–Ga ternary system at 500 ◦ C. Compounds
Space group
Structure type
Lattice parameters (nm) a
Gd Fe Ga GdGa2 GdGa Gd3 Ga2 Gd5 Ga3 GdFe2 GdFe3 Gd2 Fe17 ␣-Fe3 Ga
P63 /mmc ¯ Im3m
Reference b
c
Cmcm P6/mmm Cmcm P4/mcm P4/ncc ¯ Fd3m ¯ R3m ¯ R3m ¯ P 3m
La W Ga AlB2 BCr Gd3 Ga2 Ba5 Si3 Cu2 Mg Be3 Nb Th2 Zn17 AuCu3
0.3402 0.29315 0.290 0.4223 0.4341 1.1666 0.7716 0.753 0.5148 0.8540 0.3679
1.1047
␣-Fe6 Ga5
C2/m
Fe6 Ge5
1.0058 ˇ = 109.33◦
0.7946
0.7747
[10]
Fe3 Ga4
C2/m
Fe3 Ga4
1.0091 ˇ = 106.67◦
0.7666
0.7866
[10]
FeGa3
P42 /mnm
FeGa3
0.62628
GdFe6 Ga6 GdFe5.3 Ga6.7 GdFe6 Ga6 GdFe5 Ga7
Immm
ScFe6 Ga6
I4/mmm
ThMn12
0.856761 (6) 0.85676 (9) 0.864 (1) 0.8651 (1)
0.813 1.102
0.317 0.4137 0.4066 1.5061 1.4223 2.462 1.2428
[10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10]
0.65559 0.86775 (5) 0.86960 (9)
0.50786 (2) 0.50782 (5) 0.5074 (7) 0.50934 (6)
[10] This work [10] This work
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Fig. 3. Rietveld refinements for XRD pattern of the ternary compounds (a) GdFe5.3 Ga6.7 and (b) GdFe5 Ga7 .
Table 2 Structure, refined parameters for the ternary compounds GdFe5.3 Ga6.7 and GdFe5 Ga7 . Sample
GdFe5.3 Ga6.7
GdFe5 Ga7
Space group Structure type Cell parameters (nm) Volume of unit cell (nm3 ) Calculated density (gcm−3 ) Reliability factors
Immm ScFe6 Ga6 a = 0.85676 (9), b = 0.86960 (9) c = 0.50782 (5) V = 0.37834 (7) 8.237 (2) Rp = 8.59, Rwp = 12.26 Rexp = 9.74, GOF = 1.26
I4/mmm ThMn12 a = 0.8651 (1), c = 0.50934 (6) V = 0.3804 (1) 8.00 (3) Rp = 9.94, Rwp = 14.26 Rexp = 10.90, GOF = 1.31
Atomic parameters GdFe5.3 Ga6.7 (ScFe6 Ga6 -type, Immm) Atom Position Gd 2a (0, 0, 0) Ga 4e (x, 0, 0), x = 0.337 (1) Fe 4f (x, 1/2, 0), x = 0.263 (1) Ga 4 g (0, y, 0), y = 0.341 (1) Ga 4 h (0, y, 1/2), y = 0.807 (1) 8k (1/4, 1/4, 1/4) Fe0.825 Ga0.175
Occ. 1 1 1 1 1 1
GdFe5 Ga7 (ThMn12 -type, I4/mmm) Atom Position Gd 2a (0, 0, 0) 8f (1/2, 1/2, 1/2) Fe0.85 Ga0.15 Ga 8i (x, 0, 0), x = 0.3409 (7) 8j (x, 1/2, 0), x = 0.2815 Fe0.4 Ga0.6
Occ. 1 1 1 1
D.C. Liu et al. / Journal of Alloys and Compounds 479 (2009) 134–139
compounds in the Gd–Fe–Ga ternary system at 500 ◦ C are listed in Table 1. A structural transformation from hexagonal Th2 Ni17 -type structure to rhombohedral Th2 Zn17 -type structure was found for substitution of Ga for Fe in the compounds Er2 Fe17 , Tm2 Fe17 and Dy2 Co17 [12,13]. The XRD patterns for Gd2 Fe17−x Gax in this work indicate that this compound crystallizes in rhombohedral Th2 Zn17 -type structure. Substitution of Ga for Fe in Gd2 Fe17 keeps the rhombohedral Th2 Zn17 -type structure, but increases its lattice parameters. The XRD patterns for Gd2 Fe17 and Gd2 Fe10 Ga7 are shown in Fig. 1 as an example. The pattern of Gd2 Fe10 Ga7 shifts to lower angle obviously as comparing to that of Gd2 Fe17 , indicating the lattice parameters of this compound increases with Ga substitution. Fig. 2 show the lattice parameters a, c (Fig. 2(a)) and unit cell volume V (Fig. 2(b)) of Gd2 Fe17−x Gax with various Ga obtained by the Rietveld refinements of Gd2 Fe17−x Gax structures using Topas V3.0 software. It can be seen that the lattice parameters a and c increase linearly with increasing Ga content up to x = 5.5 (29 at.% Ga). With further increasing Ga content x to 8.0 (42 at.% Ga), the lattice parameter c increases slightly, whereas the lattice parameter a increases more quickly than that for x less than 5.5. However, the unit-cell volume of the Gd2 Fe17−x Gax compound increases linearly with increasing Ga content x from 0 to 8.0 (from 0 to 42 at.% Ga) and keeps constant for Ga content more than 45 at.% Ga. The maximum solid solubility of Ga in Gd2 Fe17 is determined to be 44.3 at.% at 500 ◦ C. The same behavior was found for substitution of Ga for Co in the compound Dy2 Co17−x Gax [13]. Two ternary compounds GdFe5.3 Ga6.7 and GdFe5 Ga7 are found in this system at 500 ◦ C. The Rietveld refinements, shown in Fig. 3 and Table 2, confirmed that the compounds crystallize in the orthorhombic ScFe6 Ga6 -type structure (Immm) for GdFe5.3 Ga6.7 (Fig. 3(a)) but in the tetragonal ThMn12 -type structure (I4/mmm) for GdFe5 Ga7 (Fig. 3(b)). The low agreement factors and low the Goodness-of-fits (GOFs) (see Table 2) indicate that the Rietveld refinements are satisfactory. The single phase with ScFe6 Ga6 -type
137
structure was found in the sample with composition of GdFe5.3 Ga6.7 only and no movement for the XRD pattern of this phase was found in other alloys located in the two- or three-phase regions, containing this phase. It indicates that the homogeneity range of this phase is very small. The homogeneity range for the compound GdFe5 Ga7 was determined to be from 53.8 to 59.2 at.% Ga (from GdFe5 Ga7 to GdFe4.3 Ga7.7 ). The lattice parameters a, c and unit cell volume V for this compound with various Ga, obtained by the Rietveld refinements of its structure using Topas V3.0 software, are shown in Fig. 4. The variations of the lattice parameters are similar with those of the compound Gd2 Fe17−x Gax . The lattice parameters a and c increase linearly with increasing Ga content from 53.8 to 57.7 at.% Ga (from GdFe5 Ga7 to GdFe4.3 Ga7.5 ), but the lattice parameter c increases slightly while the lattice parameter a increases more quickly with further increasing Ga content to 59.0 at.% Ga (GdFe4.34 Ga7.66 ). The unit-cell volume of this compound increases linearly with increasing Ga content for the composition range from 53.8 to 59.0 at.% Ga ((from GdFe5 Ga7 to GdFe4.34 Ga7.66 ) and keeps constant for Ga content more than 59.0 at.% Ga. The orthorhombic ScFe6 Ga6 -type structure has a close relationship with the tetragonal ThMn12 -type structure. When the atoms at 8i and 8j sites in the tetragonal ThMn12 -type structure become order, the structure transforms to the orthorhombic ScFe6 Ga6 type structure with the 8i (ThMn12 ) → the 4e, 4 g (ScFe6 Ga6 ) and the 8j (ThMn12 ) → the 4f, 4 h (ScFe6 Ga6 ). The XRD patterns for the two compounds are very similar but more superlattice lines appeared obviously in the XRD pattern of GdFe5.3 Ga6.7 (Fig. 3(a)) as compared with that of GdFe5 Ga7 (Fig. 3(b)). The ternary gallide GdFe6 Ga6 was reported to be the tetragonal ThMn12 -type structure at high temperature (800 ◦ C) while orthorhombic ScFe6 Ga6 -type structure at lower temperature (600 ◦ C) [14]. This work confirms this phase transformation can be performed by changing the Ga content in limit range composition. The substitution of Ga for Fe in GdFe5.3 Ga6.7 is benefit for the phase transformation from the orthorhombic ScFe6 Ga6 -type structure to the tetragonal ThMn12 type structure. 3.2. Isothermal section of the Gd–Fe–Ga ternary system at 500 ◦ C By comparing and analyzing the X-ray diffraction patterns of about 130 samples, and identifying the phases present in each sample, we determined the phase equilibria in the Gd–Fe–Ga ternary system at 500 ◦ C and thus constructed its isothermal section,
Fig. 4. Variations of the lattice parameters a, c (a) and the unit cell volume V (b) of GdFe5−x Ga7+x with Ga content (in at.% Ga).
Fig. 5. Isothermal section of the phase diagram of the Gd–Fe–Ga ternary system at 500 ◦ C.
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Fig. 6. XRD pattern for the samples with the composition of (a) Gd47 Fe45 Ga8 (in at.%) located in the GdFe2 + Gd5 Ga3 + Gd three-phase region; (b) Gd48 Fe23 Ga29 located in the Gd3 Ga2 + GdGa + GdFe3 three-phase region; (c) Gd34 Fe23 Ga43 located in the Gd3 Ga2 + GdGa + Gd2 Fe17 three-phase region and (d) Gd10 Fe19 Ga71 located in the GdGa2 + FeGa3 + GdFe5 Ga7 three-phase region.
shown in Fig. 5. It is composed of 16 single-phase regions, 31 twophase regions and 16 three-phase regions. The single-phase regions are Gd, Fe, Ga (liquid), GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , ␣-Fe3 Ga, ␣-Fe6 Ga5 , Fe3 Ga4 , FeGa3 , GdFe5.3 Ga6.7 and GdFe5 Ga7 . The X-ray diffraction patterns for some representative samples located in some three-phase regions are shown in Fig. 6(a) for the sample with the composition of Gd47 Fe45 Ga8 (in at.%) (label with A in Fig. 5) located in the GdFe2 + Gd5 Ga3 + Gd three-phase region; Fig. 6(b) for the sample with the composition of Gd48 Fe23 Ga29 (label with B in Fig. 5) located in the Gd3 Ga2 + GdGa + GdFe3 three-phase region; Fig. 6(c) for the sample with the composition of Gd34 Fe23 Ga43 (label with C in Fig. 5) located in the Gd3 Ga2 + GdGa + Gd2 Fe17 three-phase region; and Fig. 6(d) for the sample with the composition of Gd10 Fe19 Ga71 (label with D in Fig. 5) located in the GdGa2 + FeGa3 + GdFe5 Ga7 threephase region, respectively. One ternary compound LaFe2 Ga8 in La–Fe–Ga ternary system, two compounds CeFe2 Ga8 and CeFe6 Ga7 in Ce–Fe–Ga ternary system, and more than 6 compounds in Sc–Fe–Ga or Er–Fe–Ga ternary systems were reported [6]. However, only two ternary compounds, GdFe5.3 Ga6.7 and GdFe5 Ga7 were found in the Gd–Fe–Ga ternary system at 500 ◦ C. The homogeneity ranges of the single-phase regions were determined using phase disappear method, lattice parameter method and on the basis of the movement of the XRD pattern of the phase in the sample with different compositions. Fig. 7(a) show the variation of the lattice parameter a of GdFe2−x Gax with Ga content, from which we obtain that the maximum solid solubility of Ga in GdFe2 is 9.2 at.% at 500 ◦ C. In the same way, the maximum solid solubilities of Ga in GdFe3 , Gd2 Fe17 are determined to be 9.0 (Fig. 7(b)), and 44.3 at.% at 500 ◦ C, respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in
Fig. 7. Variations of the lattice parameters (a) a of GdFe2−x Gax and (b) a and c of GdFe3−x Gax with Ga content (in at.% Ga) for determination of the maximum solid solubilities of Ga in GdFe2 and GdFe3 .
D.C. Liu et al. / Journal of Alloys and Compounds 479 (2009) 134–139
this compound is very small. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. The narrow range of homogeneity in the binary compounds in the Fe–Ga system were observed, which was in good agreement with that reported in ref. [6]. Very limited solid solutions were measured in the other compounds. 4. Conclusion We have investigated and constructed the isothermal section of the Gd–Fe–Ga ternary system at 500 ◦ C. It is consisted of 16 single-phase regions, 31 two-phase regions and 16 three-phase regions. Two ternary compounds, GdFe5.3 Ga6.7 with orthorhombic ScFe6 Ga6 -type structure (Immm) and GdFe5 Ga7 with the tetragonal ThMn12 -type structure (I4/mmm) were found in this isothermal section. The maximum solid solubility of Ga in GdFe2 , GdFe3 , Gd2 Fe17 are determined to be 9.2, 9.0, and 44.3 at.% respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in this compound can not be observed. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 50871070), Shenzhen Science and Technology Research Grant (No. 200726).
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The phase relations in the Gd–Fe–Ga ternary system at 500 ◦ C D.C. Liu, J.Q. Li ∗ , M. Ouyang, F.S. Liu, W.Q. Ao College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 10 December 2008 Accepted 15 December 2008 Available online 25 December 2008 Keywords: Rare earth alloys and compounds Phase diagram X-ray diffraction
a b s t r a c t The isothermal section (500 ◦ C) of the phase diagram of the Gd–Fe–Ga ternary system was investigated by X-ray powder diffraction analysis. Eleven binary compounds, GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , Fe3 Ga, Fe6 Ga5 , Fe3 Ga4 and FeGa3 , have been confirmed. Two ternary compounds, GdFe5.3 Ga6.7 and GdFe5 Ga7 , were found in this ternary system at 500 ◦ C. The compound GdFe5.3 Ga6.7 is orthorhombic ScFe6 Ga6 -type structure (space group Immm) with a = 0.8567 (9), b = 0.86960 (9) and c = 0.50782 (5) nm, while the compound GdFe5 Ga7 is tetragonal ThMn12 -type structure (space group I4/mmm) with a = 0.8651(1) and c = 0.50934 (6) nm. The isothermal section at 500 ◦ C consists of 16 single-phase regions, 31 two-phase regions and 16 three-phase regions. The maximum solid solubilities of Ga in GdFe2 , GdFe3 , Gd2 Fe17 are 9.2, 9.0, and 44.3 at.% respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in this compound is very small. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. Very limited solid solutions were measured in the other compounds. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental detail
Gallium has unique interaction with rare-earth (R)–transition metal (T) alloys. The studies of the phase diagrams of the Ho–Co–Ga, Sm–Co–Ga, Er–Fe–Ga and Sc–Fe–Ga ternary systems show rich ternary compounds or substitution derivatives in R–T–Ga systems [1]. The compound RFe12−x Gax which derives from the ThMn12 type structure, such as RFe6 Ga6 , is stable while RFe12 is not existed in the R–Fe binary system [2]. For the compound R2 Fe17−x Gax (R = Y, Sm Gd, Tb, Ho and Tm), the values of its Tc increase with Ga concentration strongly [3]. Systematic studies of the gallium substituted series R2 Co17−x Gax have shown that the phase relations and structural properties change strongly as a function of Ga content. As the gallium content increases, the hexagonal Th2 Ni17 -type structure transforms into the rhombohedral Th2 Zn17 -type structure, the Curie temperature and the saturation magnetization decrease [4]. From the reported phase diagram of the R–T–Ga ternary system, we found that the phases and phase relationships in theses system are different with different rare earth. A phase diagram plays an important role for materials researches. The investigation iron-rich corner of the Yb–Fe–Ga phase diagram was currently reported [5]. The investigation of the Gd–Fe–Ga has not been reported. In this work, we investigated the isothermal section of this ternary system at 500 ◦ C.
The polycrystalline Gd–Fe–Ga alloys were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in pure argon atmosphere. Gadolinium, Iron and Gallium with purity of 99.99% or higher were used as starting materials. Titanium was used as an O getter during the melting process. The alloys were re-melted at least three times in order to ensure complete fusion and homogeneous composition. The melted alloy buttons were sealed in evacuated quartz tubes containing titanium chips as an O getter placed in a resistance furnace for homogenization. The homogenization was performed at 800 ◦ C for 20 days for the alloys near Fe corner with Gd less than 33.3 at.% and Ga less than 50 at.%, and at 650 ◦ C for 25 days for the other alloys. They were cooled from their homogenization temperatures to 500 ◦ C, kept at 500 ◦ C for 3 days and then quenched in liquid nitrogen. About 130 samples in total were prepared for this investigation. X-ray powder diffraction (XRD) data was collected by a Bruker D8 Advance SS/18 kW diffractometer with Cu K␣ radiation. JADE 5.0 and Topas 3.0 softwares were used for phase analysis and structure refinement.
∗ Corresponding author. E-mail address: [email protected] (J.Q. Li). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.057
3. Results and discussion 3.1. Phase analysis The phase diagrams of the Gd–Fe, Gd–Ga and Fe–Ga binary systems were reported [6]. Four binary compounds, Gd2 Fe17 , Gd6 Fe23 , GdFe3 and GdFe2 exist in the Gd–Fe phase diagram, formed peritectically at 1335, 1280, 1155 and 1080 ◦ C, respectively. The compound Gd6 Fe23 was drawn with dot line, indicating this compound needs to be confirmed. In the work by Copeland et al. [7]. and Spedding et al. [8], only three intermetallic compounds were found, i.e. Fe17 Gd2 , Fe3 Gd and Fe2 Gd. Based on the Gd–Fe phase diagram reported in ref. [9], the compound Gd6 Fe23 formed peritectically at 1280 ◦ C but decomposes into Fe17 Gd2 and Fe3 Gd at about 1150 ◦ C.
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Fig. 1. X-ray diffraction patterns for the compounds (a) Gd2 Fe17 and (b) Gd2 Fe10 Ga7 with Th2 Zn17 -type structure.
Five binary compounds, Gd5 Ga3 , Gd3 Ga2 , GdGa, GdGa2 and GdGa6 exist in the Gd–Ga phase diagram. The compound GdGa2 melts congruently at 1400 ◦ C, while the compounds Gd5 Ga3 , Gd3 Ga2 , GdGa and GdGa6 form peritectically at 1010, 1110, 1195 and 406 ◦ C, respectively. A large homogeneity range from 22 to 33.3 at.% Ga is shown in GdGa2 . Four intermediate phases exist in the Fe–Ga system: FeGa3 , Fe3 Ga4 , Fe6 Ga5 and Fe3 Ga, with polymorphic modifications for the last two: ␣-Fe3 Ga (cubic, Pm3m) for T < 619 ◦ C and -Fe3 Ga (hexagonal, P63 /mmc) for T > 619 ◦ C and ␣-Fe6 Ga5 (mono¯ clinic, C2/m) for T > 778 ◦ C and -Fe6 Ga5 (rhombohedral, R3m) for T < 778 ◦ C. All the binaries belonging to the Fe–Ga system except FeGa3 (tetragonal, P4/mnm), show a narrow range of homogeneity: ␣-Fe3 Ga (26–29 at.% Ga), ␣-Fe6 Ga5 (44.5–45.5 at.% Ga) and Fe3 Ga4 (57–58 at.% Ga). The structures, ScFe6 Ga6 -type (Immm) and ThMn12-type (I4/mmm), for the ternary compound GdFe6 Ga6 were reported in the Gd–Fe–Ga ternary system [10]. X-ray diffraction analysis for patterns of about 130 samples in this work confirmed the existence of the eleven binary compounds, GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , ␣-Fe3 Ga, ␣-Fe6 Ga5 , Fe3 Ga4 and FeGa3 , in the Gd–Fe–Ga ternary system at 500 ◦ C, which is in good agreement with those reported in ref. [6]. In this work and our previous work [11], we found the XRD pattern of the sample with composition of Gd6 Fe23 consists of those of Gd2 Fe17 and GdFe3 (see Fig. 1 in ref. [11]). We believed
Fig. 2. Variations of the lattice parameters a, c (a) and the unit cell volume V (b) of Gd2 Fe17−x Gax with Ga content (in at.% Ga).
that our result was in agreement with that reported in ref. [9]: the compound Gd6 Fe23 decomposes into Fe17 Gd2 and Fe3 Gd at higher temperature and should not existed at 500 ◦ C. Pure gallium is the only liquid phase at 500 ◦ C. A trace of the XRD pattern of GdGa6 appears in the XRD patterns of the alloys near Ga corner, but GdGa6 should not exist at 500 ◦ C since it decomposes at the temperature higher than 406 ◦ C. The alloys near Ga corner may contain the equilibrium liquid phase at 500 ◦ C. A small amount of GdGa6 phase may deposit from the liquid phase during the quenching. The crystal structures and lattice parameter data for the
Table 1 Crystal structures and lattice parameter data for the compounds in Gd–Fe–Ga ternary system at 500 ◦ C. Compounds
Space group
Structure type
Lattice parameters (nm) a
Gd Fe Ga GdGa2 GdGa Gd3 Ga2 Gd5 Ga3 GdFe2 GdFe3 Gd2 Fe17 ␣-Fe3 Ga
P63 /mmc ¯ Im3m
Reference b
c
Cmcm P6/mmm Cmcm P4/mcm P4/ncc ¯ Fd3m ¯ R3m ¯ R3m ¯ P 3m
La W Ga AlB2 BCr Gd3 Ga2 Ba5 Si3 Cu2 Mg Be3 Nb Th2 Zn17 AuCu3
0.3402 0.29315 0.290 0.4223 0.4341 1.1666 0.7716 0.753 0.5148 0.8540 0.3679
1.1047
␣-Fe6 Ga5
C2/m
Fe6 Ge5
1.0058 ˇ = 109.33◦
0.7946
0.7747
[10]
Fe3 Ga4
C2/m
Fe3 Ga4
1.0091 ˇ = 106.67◦
0.7666
0.7866
[10]
FeGa3
P42 /mnm
FeGa3
0.62628
GdFe6 Ga6 GdFe5.3 Ga6.7 GdFe6 Ga6 GdFe5 Ga7
Immm
ScFe6 Ga6
I4/mmm
ThMn12
0.856761 (6) 0.85676 (9) 0.864 (1) 0.8651 (1)
0.813 1.102
0.317 0.4137 0.4066 1.5061 1.4223 2.462 1.2428
[10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10]
0.65559 0.86775 (5) 0.86960 (9)
0.50786 (2) 0.50782 (5) 0.5074 (7) 0.50934 (6)
[10] This work [10] This work
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Fig. 3. Rietveld refinements for XRD pattern of the ternary compounds (a) GdFe5.3 Ga6.7 and (b) GdFe5 Ga7 .
Table 2 Structure, refined parameters for the ternary compounds GdFe5.3 Ga6.7 and GdFe5 Ga7 . Sample
GdFe5.3 Ga6.7
GdFe5 Ga7
Space group Structure type Cell parameters (nm) Volume of unit cell (nm3 ) Calculated density (gcm−3 ) Reliability factors
Immm ScFe6 Ga6 a = 0.85676 (9), b = 0.86960 (9) c = 0.50782 (5) V = 0.37834 (7) 8.237 (2) Rp = 8.59, Rwp = 12.26 Rexp = 9.74, GOF = 1.26
I4/mmm ThMn12 a = 0.8651 (1), c = 0.50934 (6) V = 0.3804 (1) 8.00 (3) Rp = 9.94, Rwp = 14.26 Rexp = 10.90, GOF = 1.31
Atomic parameters GdFe5.3 Ga6.7 (ScFe6 Ga6 -type, Immm) Atom Position Gd 2a (0, 0, 0) Ga 4e (x, 0, 0), x = 0.337 (1) Fe 4f (x, 1/2, 0), x = 0.263 (1) Ga 4 g (0, y, 0), y = 0.341 (1) Ga 4 h (0, y, 1/2), y = 0.807 (1) 8k (1/4, 1/4, 1/4) Fe0.825 Ga0.175
Occ. 1 1 1 1 1 1
GdFe5 Ga7 (ThMn12 -type, I4/mmm) Atom Position Gd 2a (0, 0, 0) 8f (1/2, 1/2, 1/2) Fe0.85 Ga0.15 Ga 8i (x, 0, 0), x = 0.3409 (7) 8j (x, 1/2, 0), x = 0.2815 Fe0.4 Ga0.6
Occ. 1 1 1 1
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compounds in the Gd–Fe–Ga ternary system at 500 ◦ C are listed in Table 1. A structural transformation from hexagonal Th2 Ni17 -type structure to rhombohedral Th2 Zn17 -type structure was found for substitution of Ga for Fe in the compounds Er2 Fe17 , Tm2 Fe17 and Dy2 Co17 [12,13]. The XRD patterns for Gd2 Fe17−x Gax in this work indicate that this compound crystallizes in rhombohedral Th2 Zn17 -type structure. Substitution of Ga for Fe in Gd2 Fe17 keeps the rhombohedral Th2 Zn17 -type structure, but increases its lattice parameters. The XRD patterns for Gd2 Fe17 and Gd2 Fe10 Ga7 are shown in Fig. 1 as an example. The pattern of Gd2 Fe10 Ga7 shifts to lower angle obviously as comparing to that of Gd2 Fe17 , indicating the lattice parameters of this compound increases with Ga substitution. Fig. 2 show the lattice parameters a, c (Fig. 2(a)) and unit cell volume V (Fig. 2(b)) of Gd2 Fe17−x Gax with various Ga obtained by the Rietveld refinements of Gd2 Fe17−x Gax structures using Topas V3.0 software. It can be seen that the lattice parameters a and c increase linearly with increasing Ga content up to x = 5.5 (29 at.% Ga). With further increasing Ga content x to 8.0 (42 at.% Ga), the lattice parameter c increases slightly, whereas the lattice parameter a increases more quickly than that for x less than 5.5. However, the unit-cell volume of the Gd2 Fe17−x Gax compound increases linearly with increasing Ga content x from 0 to 8.0 (from 0 to 42 at.% Ga) and keeps constant for Ga content more than 45 at.% Ga. The maximum solid solubility of Ga in Gd2 Fe17 is determined to be 44.3 at.% at 500 ◦ C. The same behavior was found for substitution of Ga for Co in the compound Dy2 Co17−x Gax [13]. Two ternary compounds GdFe5.3 Ga6.7 and GdFe5 Ga7 are found in this system at 500 ◦ C. The Rietveld refinements, shown in Fig. 3 and Table 2, confirmed that the compounds crystallize in the orthorhombic ScFe6 Ga6 -type structure (Immm) for GdFe5.3 Ga6.7 (Fig. 3(a)) but in the tetragonal ThMn12 -type structure (I4/mmm) for GdFe5 Ga7 (Fig. 3(b)). The low agreement factors and low the Goodness-of-fits (GOFs) (see Table 2) indicate that the Rietveld refinements are satisfactory. The single phase with ScFe6 Ga6 -type
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structure was found in the sample with composition of GdFe5.3 Ga6.7 only and no movement for the XRD pattern of this phase was found in other alloys located in the two- or three-phase regions, containing this phase. It indicates that the homogeneity range of this phase is very small. The homogeneity range for the compound GdFe5 Ga7 was determined to be from 53.8 to 59.2 at.% Ga (from GdFe5 Ga7 to GdFe4.3 Ga7.7 ). The lattice parameters a, c and unit cell volume V for this compound with various Ga, obtained by the Rietveld refinements of its structure using Topas V3.0 software, are shown in Fig. 4. The variations of the lattice parameters are similar with those of the compound Gd2 Fe17−x Gax . The lattice parameters a and c increase linearly with increasing Ga content from 53.8 to 57.7 at.% Ga (from GdFe5 Ga7 to GdFe4.3 Ga7.5 ), but the lattice parameter c increases slightly while the lattice parameter a increases more quickly with further increasing Ga content to 59.0 at.% Ga (GdFe4.34 Ga7.66 ). The unit-cell volume of this compound increases linearly with increasing Ga content for the composition range from 53.8 to 59.0 at.% Ga ((from GdFe5 Ga7 to GdFe4.34 Ga7.66 ) and keeps constant for Ga content more than 59.0 at.% Ga. The orthorhombic ScFe6 Ga6 -type structure has a close relationship with the tetragonal ThMn12 -type structure. When the atoms at 8i and 8j sites in the tetragonal ThMn12 -type structure become order, the structure transforms to the orthorhombic ScFe6 Ga6 type structure with the 8i (ThMn12 ) → the 4e, 4 g (ScFe6 Ga6 ) and the 8j (ThMn12 ) → the 4f, 4 h (ScFe6 Ga6 ). The XRD patterns for the two compounds are very similar but more superlattice lines appeared obviously in the XRD pattern of GdFe5.3 Ga6.7 (Fig. 3(a)) as compared with that of GdFe5 Ga7 (Fig. 3(b)). The ternary gallide GdFe6 Ga6 was reported to be the tetragonal ThMn12 -type structure at high temperature (800 ◦ C) while orthorhombic ScFe6 Ga6 -type structure at lower temperature (600 ◦ C) [14]. This work confirms this phase transformation can be performed by changing the Ga content in limit range composition. The substitution of Ga for Fe in GdFe5.3 Ga6.7 is benefit for the phase transformation from the orthorhombic ScFe6 Ga6 -type structure to the tetragonal ThMn12 type structure. 3.2. Isothermal section of the Gd–Fe–Ga ternary system at 500 ◦ C By comparing and analyzing the X-ray diffraction patterns of about 130 samples, and identifying the phases present in each sample, we determined the phase equilibria in the Gd–Fe–Ga ternary system at 500 ◦ C and thus constructed its isothermal section,
Fig. 4. Variations of the lattice parameters a, c (a) and the unit cell volume V (b) of GdFe5−x Ga7+x with Ga content (in at.% Ga).
Fig. 5. Isothermal section of the phase diagram of the Gd–Fe–Ga ternary system at 500 ◦ C.
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Fig. 6. XRD pattern for the samples with the composition of (a) Gd47 Fe45 Ga8 (in at.%) located in the GdFe2 + Gd5 Ga3 + Gd three-phase region; (b) Gd48 Fe23 Ga29 located in the Gd3 Ga2 + GdGa + GdFe3 three-phase region; (c) Gd34 Fe23 Ga43 located in the Gd3 Ga2 + GdGa + Gd2 Fe17 three-phase region and (d) Gd10 Fe19 Ga71 located in the GdGa2 + FeGa3 + GdFe5 Ga7 three-phase region.
shown in Fig. 5. It is composed of 16 single-phase regions, 31 twophase regions and 16 three-phase regions. The single-phase regions are Gd, Fe, Ga (liquid), GdGa2 , GdGa, Gd3 Ga2 , Gd5 Ga3 , GdFe2 , GdFe3 , Gd2 Fe17 , ␣-Fe3 Ga, ␣-Fe6 Ga5 , Fe3 Ga4 , FeGa3 , GdFe5.3 Ga6.7 and GdFe5 Ga7 . The X-ray diffraction patterns for some representative samples located in some three-phase regions are shown in Fig. 6(a) for the sample with the composition of Gd47 Fe45 Ga8 (in at.%) (label with A in Fig. 5) located in the GdFe2 + Gd5 Ga3 + Gd three-phase region; Fig. 6(b) for the sample with the composition of Gd48 Fe23 Ga29 (label with B in Fig. 5) located in the Gd3 Ga2 + GdGa + GdFe3 three-phase region; Fig. 6(c) for the sample with the composition of Gd34 Fe23 Ga43 (label with C in Fig. 5) located in the Gd3 Ga2 + GdGa + Gd2 Fe17 three-phase region; and Fig. 6(d) for the sample with the composition of Gd10 Fe19 Ga71 (label with D in Fig. 5) located in the GdGa2 + FeGa3 + GdFe5 Ga7 threephase region, respectively. One ternary compound LaFe2 Ga8 in La–Fe–Ga ternary system, two compounds CeFe2 Ga8 and CeFe6 Ga7 in Ce–Fe–Ga ternary system, and more than 6 compounds in Sc–Fe–Ga or Er–Fe–Ga ternary systems were reported [6]. However, only two ternary compounds, GdFe5.3 Ga6.7 and GdFe5 Ga7 were found in the Gd–Fe–Ga ternary system at 500 ◦ C. The homogeneity ranges of the single-phase regions were determined using phase disappear method, lattice parameter method and on the basis of the movement of the XRD pattern of the phase in the sample with different compositions. Fig. 7(a) show the variation of the lattice parameter a of GdFe2−x Gax with Ga content, from which we obtain that the maximum solid solubility of Ga in GdFe2 is 9.2 at.% at 500 ◦ C. In the same way, the maximum solid solubilities of Ga in GdFe3 , Gd2 Fe17 are determined to be 9.0 (Fig. 7(b)), and 44.3 at.% at 500 ◦ C, respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in
Fig. 7. Variations of the lattice parameters (a) a of GdFe2−x Gax and (b) a and c of GdFe3−x Gax with Ga content (in at.% Ga) for determination of the maximum solid solubilities of Ga in GdFe2 and GdFe3 .
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this compound is very small. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. The narrow range of homogeneity in the binary compounds in the Fe–Ga system were observed, which was in good agreement with that reported in ref. [6]. Very limited solid solutions were measured in the other compounds. 4. Conclusion We have investigated and constructed the isothermal section of the Gd–Fe–Ga ternary system at 500 ◦ C. It is consisted of 16 single-phase regions, 31 two-phase regions and 16 three-phase regions. Two ternary compounds, GdFe5.3 Ga6.7 with orthorhombic ScFe6 Ga6 -type structure (Immm) and GdFe5 Ga7 with the tetragonal ThMn12 -type structure (I4/mmm) were found in this isothermal section. The maximum solid solubility of Ga in GdFe2 , GdFe3 , Gd2 Fe17 are determined to be 9.2, 9.0, and 44.3 at.% respectively. The homogeneity range of GdGa2 is from 22 to 33.3 at.% Ga in Gd–Ga side but the solid solubility of Fe in this compound can not be observed. The homogeneity range of GdFe5 Ga7 is from 53.8 to 59.2 at.% Ga. Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 50871070), Shenzhen Science and Technology Research Grant (No. 200726).
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