Formation of TbCu7-type CeFe10Zr0.8 by rapid solidification

Journal of Alloys and Compounds 569 (2013) 6–8

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Formation of TbCu7-type CeFe10Zr0.8 by rapid solidification Chen Zhou a,⇑, Frederick E. Pinkerton b, Jan F. Herbst b a b

MEDA Engineering and Technical Services LLC, 17515 W 9 Mile Road Suite 1075, Southfield, MI 48075, USA Chemical and Materials Systems Lab, General Motors R&D Center, 30500 Mound Road, Warren, MI 48090, USA

a r t i c l e

i n f o

Article history: Received 15 March 2013 Accepted 20 March 2013 Available online 27 March 2013 Keywords: Curie temperature Melt-spinning Rare earth magnets Rietveld method Space group

a b s t r a c t We report the discovery of a new ternary compound prepared by melt spinning induction melted ingot of nominal composition CeFe11Zr. The sample melt spun at vs = 25 m/s exhibits the hexagonal TbCu7-type structure of space group P6/mmm. Through fitting the experimental X-ray diffraction pattern by Rietveld method, we have successfully derived the crystal structure of the new compound melt spun at vs = 25 m/s to be CeFe10Zr0.8. Subsequent density function theory calculation fully supports the chemical stability of the new ternary compound. Annealing test showed that the melt spun CeFe10Zr0.8 is stable up to 700 °C and annealing at higher temperature would cause it to decompose into hexagonal Ce2Fe17-type structure and ZrFe2. The Curie temperature measurement found that CeFe10Zr0.8 boasts a Tc = 181 °C, which is higher than the Tc values of all known Ce–Fe binary compounds, and 30 °C higher than that of Ce2Fe14B. These interesting properties stimulate continued search for new Ce-based permanent magnets that could be a cost effective solution to engineering needs in the future. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The stringent supply of critical rare-earth elements in recent years has spurred research effort in revisiting Ce-based intermetallic compounds for potential permanent magnet applications. The binary compounds CeFe2, CeFe5, CeFe7, and Ce2Fe17 exist but have Curie temperatures Tc below 300 K [1–3]. Another known compound system found in the literature is the CeFe12xMx (where M usually represents the transition metal elements) with the tetragonal ThMn12-type structure (also known as the 1:12 phase) [4]. The CeFe12 phase does not exist by itself. In order to form the 1:12 structure, a transition metal other than Fe must be added. Out of the vast number of available elements that have been identified to form the RFe12xMx (R represents a general rare-earth element), only Mo, Ti, V, and Cr have been reported to form the 1:12 structure with Ce [5]. Since CeFe11Ti has been reported to exist and Zr is in the same column of the periodic table as Ti, we were motivated to prepare samples having the CeFe11Zr stoichiometry by melt spinning at surface wheel speeds from 5 m/s to 25 m/s. The fast quenching speed of melt spinning enables bypassing the intermediate phases, which in some cases eliminates the necessity of time consuming annealing. Rather than forming the 1:12 structure, X-ray diffraction reveals that ribbons spun at 5 m/s form Ce2Fe17. The sample melt spun at vs = 25 m/s, however, forms a previously unknown Ce–Fe–Zr compound having the hexagonal TbCu7-type structure

(1:7) and Tc = 181 °C. From the prototype crystal structure of TbCu7 we have derived the detailed crystal structure and chemical composition of the new compound by Rietveld refinement of the experimental X-ray diffraction pattern. In addition, density functional theory (DFT) calculations have been performed to elucidate the role of Zr in the formation of the new ternary compound. 2. Experimental procedures Pure elements of Ce (pieces 99.8%), Fe (granules 99.98%) and Zr (lump 99.2%) were weighed according to the nominal composition CeFe11Zr. The weighed elements were induction melted in an Ar protected chamber. The soak time after melting the elements into liquid phase was about 3–5 min to ensure chemical homogeneity by induction stirring. The as-cast ingot was broken into smaller pieces and loaded into quartz ampoules with a 0.65 ± 0.01 mm orifice at the bottom where the molten alloy was ejected. The collected ribbons were ball milled into powder in a Spex 8000M mill. The sample melt spun at vs = 25 m/s was annealed from 450 °C to 900 °C at 50 °C step width for 5 min in the Perkin Elmer TGA7 system to test the chemical stability. Tc was obtained from thermogravimetric curves during heating and cooling in the same Perkin Elmer TGA7 system under magnetic force. Density was measured on powders using a Micromeritics AccuPyc II gas pycnometer. X-ray diffraction patterns for the as-spun and annealed samples were collected in a Rigaku Rapid II system using Mo Ka as the radiation source from 10° to 60° 2h. In addition, X-ray diffraction patterns from 10° to 90° 2h were collected for the asspun vs = 25 m/s sample in a Bruker D8 ADVANCE diffractometer with DaVinci design system using Cu Ka as the radiation source in order to obtain better resolution for the structural refinement.

3. Results and discussion ⇑ Corresponding author. Tel.: +1 586 441 8385. E-mail addresses: [email protected], [email protected] (C. Zhou). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.175

In general the 1:7 structure tends to form at higher wheel speed (equivalent to higher quenching rate) in melt spinning. For

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C. Zhou et al. / Journal of Alloys and Compounds 569 (2013) 6–8

instance, Katter et al. have reported SmFe9 with the 1:7 structure was formed at vs P 15 m/s and that a lower quench rate led to a 2:17 structure [6]. Pinkerton et al. have also reported that wheel speed greater than 17.5 m/s suppresses the 1:12 structure in NdFe10Mo2 and at vs = 30 m/s the phase completely transforms to the 1:7 structure [7]. Buschow et al. reported that TbCu7 belongs to the hexagonal system of space group P6/mmm. They derived the 1:7 unit cell from its TbCu5 parent structure by replacing some of the Tb atoms with a pair of Cu atoms on adjacent 2e sites while conserving the TbCu5 structure [8]. In our case, we placed Ce at Tb sites and Fe at the Cu sites described by Buschow et al. Placing Zr on the rare-earth site resulted in poor fitting, so it is an unlikely case. Thus it is reasonable to assume that Zr occupies the transition metal sites. The initial occupancies of Fe and Zr were calculated based on site multiplicity and nominal composition. The atomic positions and site occupancies for each atom were allowed to change in the X-ray profile fitting until convergence was reached. The results indicate that Zr preferentially occupies the 2e site and the Fe occupancy at the 2e site eventually converges to zero. Likewise, the Zr occupancies of the 2c and 3g sites are very small compared to the occupancy of the 2e site, while the Fe atoms approach nearly full occupancy at the 2c and 3g sites. Thus we conclude that the new compound features the 1:7 structure with Zr preferentially occupying the 2e site while Fe fills the 2c and 3g sites. Fig. 1 shows the fit to the X-ray diffraction pattern. The only impurity phase found is CeFe2, accounting for 8.16 wt.% of the sample. The overall calculated density is 7.59 g/cm3, which is in reasonable agreement with 7.82 g/cm3 measured from the pycnometer. Table 1 provides the atomic positions, site occupancies and the estimated chemical composition, CeFe10Zr0.8. Table 2 summarizes the refinement results. We have applied DFT to elucidate the role of Zr in the formation of the TbCu7-type phase. Relevant enthalpies of formation DH have been calculated using the total electronic energies E derived with the Vienna ab initio Simulation Package (VASP), which implements DFT [9] within a plane wave basis set [10,11]. Potentials constructed by the projector-augmented wave (PAW) method were employed for the elements [12,13]; the generalized gradient approximation of Perdew et al. [14,15] in conjunction with the interpolation formula of Vosko et al. [16] was used for the exchange–correlation energy functional. Since our preliminary objective was to synthesize CeFe11Zr in the ThMn12 structure, we first

Table 1 Atomic positions, site occupancies and estimated composition for Ce–Fe–Zr of 1:7 structure. Atom

Ce1a Fe2c Fe3g Zr2e Composition

TbCu7 P6/mmm (191) X

Y

Z

r

0.000 0.333 0.500 0.000 CeFe10Zr0.8

0.000 0.667 0.000 0.000

0.000 0.000 0.500 0.319

0.504 1.000 1.000 0.204

Table 2 Fitting results, structural and physical parameters for Ce–Fe–Zr ternary compound of 1:7 structure. Density in the table is the overall density of the as-spun sample, which includes the contribution from the impurity phase CeFe2. Prototype Space group Rp Rwp a (Å) c (Å) Cell volume (Å3) Density (g/cm3) wt.%

TbCu7 P6/mmm (191) 3.09 4.12 4.869 4.169 85.592 7.59 91.84

explored materials of that type with DFT. The ThMn12 crystal structure is tetragonal with the Th (Mn) ions occupying 2a (8i, 8j, 8f) sites in the I4/mmm space group (No. 139) having Z = 2 formula units per unit cell. In the case of the progenitor compound CeFe12 we obtain

DH½CeFe12 ðThMn12 -typeÞ  EðCeFe12 Þ  EðCeÞ  12EðFeÞ ¼ 11 kJ=mole CeFe12 

ð1Þ

This quantity represents the standard enthalpy of CeFe12 formation at zero temperature in the absence of zero point energy contributions. Encouragingly, the positive value indicates that CeFe12 is unstable with respect to the elemental metals, consistent with the experimental observation that CeFe12 does not form under normal conditions. To assess the impact of Zr substitution we replace one of the 8i, 8j, or 8f Fe ions with Zr. The resulting structures are ordered but of lower symmetry: Zr substitution on the 8i or 8j sites generates an orthorhombic Imm2 (No. 44, Z = 2) structure while replacement of an 8f Fe ion leads to a monoclinic C2/m (No. 12, Z = 2) structure (the 8 ways of doing so in each case are equivalent) [18]. After fully relaxing these structures we find

DH½CeFe11 ZrðZr@8iÞ  E½CeFe11 ZrðZr@8iÞ  EðCeÞ  11EðFeÞ  EðZrÞ ¼ 29 kJ=mole CeFe11 Zr ¼ 2 kJ=mole atoms

ð2Þ

and, similarly,

Fig. 1. Fitting to the experimental X-ray diffraction pattern for vs = 25 m/s sample exhibiting the 1:7 structure. Ticks of row (a) mark the Bragg positions of 1:7 structure and those in row (b) mark the Bragg positions of CeFe2.

DH½CeFe11 ZrðZr@8jÞ ¼ þ42 kJ=mole CeFe11 Zr;

ð3Þ

DH½CeFe11 ZrðZr@8fÞ ¼ þ103 kJ=mole CeFe11 Zr

ð4Þ

It is clear that Zr not only prefers to occupy the 8i site, but 8i occupation alone affords thermodynamic stability (DH < 0). Guided by our experimental work, we turn now to TbCu7-type materials. The crystal structure of TbCu7 established by Buschow and van der Goot is hexagonal P6/mmm (No. 191) with fractionally occupied Tb (1a) and Cu (2e) sites and filled Cu (2c) and Cu (3g) positions [8]. We construct an ordered approximant for CeFe11Zr

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C. Zhou et al. / Journal of Alloys and Compounds 569 (2013) 6–8

Comparison of Eqs. (2) and (5) indicates that the CeFe11Zr stoichiometry is more stable in the ThMn12 than in the TbCu7 structure type, but Eq. (6) shows that TbCu7-type CeFe10Zr is more stable than both CeFe11Zr structures on either a per formula unit or per atom basis. Moreover, a calculation for CeFe11 using the same CeFe10Zr structural approximant with Fe replacing Zr yields

DH½CeFe11 ðTbCu7 -typeÞ ¼ þ214 kJ=mole CeFe11 ¼ þ18 kJ=mole atoms;

Fig. 2. X-ray diffraction patterns of the vs = 25 m/s sample (a) as-spun; (b) annealed at 700 °C for 5 min; (c) annealed at 900 °C for 5 min. Ticks of 1:7 mark the Bragg positions of the Ce–Fe–Zr ternary compound of 1:7 structure. The diffraction patterns shown in the figure are from Mo radiation source.

ð7Þ

making it clear that Zr is crucial to the thermodynamic stability of this structure as well. Thus, our DFT results fully support the CeFe10Zr0.8 stoichiometry obtained from experiment. Fig. 2 shows the annealing results. The sample remains stable after annealing at 700 °C for 5 min. Annealing at higher temperature transforms the sample to a hexagonal Ce2Fe17-type Ce–Fe–Zr structure and ZrFe2. The Curie temperature for CeFe10Zr0.8 was found to be Tc = 181 °C (see Fig. 3). This value is much higher than Tc = 38 °C of CeFe2 [1], Tc = 181 °C for CeFe7 [17], Tc = 35 °C for rhombohedral Ce2Fe17 [1], and even 30 °C higher than that for Ce2Fe14B, which is an isomorph of Nd2Fe14B and contains 40 at.% more Ce. 4. Summary We have discovered a new Ce–Fe–Zr ternary compound by melt spinning. The sample melt spun at vs = 25 m/s forms the TbCu7type structure. Through Rietveld analysis of the experimental Xray diffraction pattern, we estimated that the chemical composition is CeFe10Zr0.8, which is fully supported by our DFT calculations. Tc = 181 °C is much higher than all known binary Ce–Fe compounds and is also 30 °C higher than Ce richer Ce2Fe14B. Acknowledgements This project is funded by ARPA-E under the grant number 04721526. The authors acknowledge Martin Meyer, Misle Tessema, and Eric Poirier for technical assistance. Chen Zhou also thanks James Salvador for helpful discussion. References

Fig. 3. Thermogravimetric curves during heating and cooling under magnetic force. Curie temperature Tc = 181 °C was the average of the two Curie temperatures from heating and cooling.

by (a) placing Ce on the site 1a and filling the 2c, 2e (0 0 z), and 3g sites with Fe; (b) doubling the cell along the c-axis; and (c) removing the Ce (0 0 ½), Fe (0 0 z/2), and Fe (0 0 z/2) sites, and replacing Fe at (0 0 z/2 + ½) with Zr. This yields an ordered hexagonal P6mm (No. 183; Z = 1) CeFe11Zr structure. Removing the Fe (0 0 z/2 + ½) ion as well leads to an ordered model for CeFe10Zr, again in the P6mm space group, to approximate the CeFe10Zr0.8 stoichiometry determined from our Rietveld analysis of the diffraction data. The relaxed total energies yield

DH½CeFe11 ZrðTbCu7 -typeÞ ¼ 15 kJ=mole CeFe11 Zr ¼ 1 kJ=mole atoms;

ð5Þ

DH½CeFe10 ZrðTbCu7 -typeÞ ¼ 62 kJ=mole CeFe10 Zr ¼ 5 kJ=mole atoms

ð6Þ

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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