Crystal structure and phase relations of Pr2Fe14B-La2Fe14B system

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016, P. 1121

Crystal structure and phase relations of Pr2Fe14B-La2Fe14B system YAO Qingrong (姚青荣)1,2,*, SHEN Yihao (沈逸豪)1, YANG Pengcheng (杨鹏程)1, ZHOU Huaiying (周怀营)1,3, RAO Guanghui (饶光辉)1, DENG Jianqiu (邓建秋)1, WANG Zhongmin (王仲民)1, ZHONG Yan (钟 燕)1,3,4 (1. School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China; 2. School of Materials Science and Engineering, Central South University, Changsha 410083, China; 3. Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China; 4. Guangxi Experiment Center of Information Science, Guilin 541004, China) Received 22 March 2016; revised 24 June 2016

Abstract: The crystal structure and phase relations of the Pr2Fe14B-La2Fe14B system were investigated by X-ray powder diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The crystal structure parameters were determined by full-profile Rietveld refinements. The results revealed that all alloys of (Pr1–xLax)2Fe14B crystallized the Nd2Fe14B-type structure with the space group P42/mnm and formed a continuous solid solutions between x=0.0 and 1.0. The lattice parameter a, c, unit-cell volume V and c/a ratio increased linearly with the La concentration. Determined by thermogravimetry analysis, the Curie temperature (TC), phase transition temperature and melting temperature of (Pr1–xLax)2Fe14B decreased linearly upon the La content. Based on the results of DSC measurements and X-ray powder diffraction examinations, the phase diagram of the Pr2Fe14B-La2Fe14B system was built up. Keywords: crystal structure; lattice parameter; Curie temperature; thermal analysis; rare earths

Since the first and second generations of rare earth permanent magnet materials using Co resources are limited and expensive, the RE-Fe-B magnet material is prepared and becomes a new generation of high property rare earth permanent magnet after a long period of study. In the last decades, the Nd-Fe-B magnets with tetragonal Nd2Fe14B as main phase have a good application in medical devices, electronic instruments and home appliances due to their ultrahigh magnetic energy at room temperature[1–5]. However, the disadvantages of low coercivity, poor thermal stability and easy corrosion seriously limit its further development and application. To obtain high coercivities at room temperature, much research is committed to using heavy rare earth elements such as Dy, Tb to replace partial Nd in Nd2Fe14B main phase, however, the atomic magnetic moment of Tb, Dy is opposite to Fe, which can reduce the saturation magnetization and residual magnetism[6–10]. In addition, Dy, Tb and other heavy rare earth elements are a kind of strategic resources, the expensive cost force manufacture to reduce the usage of heavy rare earth from the sintered magnets[11,12]. With the aim of producing outstanding magnets with lower cost and protecting natural resources, it is vital to understand the effect of rare earth content on the coercivity and the temperature stability. The structural information helps a good understanding of the ef-

fect of doped elements on the intrinsic magnetic properties. The RE2Fe14B (RE=La, Ce, Pr, Nd) crystalline in a tetragonal structure with a space group of P42/mnm and has six crystallographically in equivalent Fe sites, two in equivalent RE sites, and a B site. There are 68 atoms in each unit cell of 2:14:1[13,14]. The magnetic properties of a permanent magnet depend not only on the compound of the principle phase, but also on the microstructure of the magnet[15]. An important tool for the control of the compound and microstructure is the phase diagram. The phase diagrams of the Sm2Fe14B-Nd2Fe14B[14] and Pr2Fe14B-Nd2Fe14B[16] systems have been reported. So far, no phase diagram of the Pr2Fe14B-La2Fe14B system was described. In the present work, we investigated the phase equilibria in the Pr2Fe14B-La2Fe14B system.

1 Experimental Master alloys of (Pr1–xLax)2Fe14B (x=0.0–1.0) with a mass of 3.0 g were prepared by arc-melting metal ingots of Pr, La, Fe and B (purity>99.99% for Pr, La, Fe and B) under an argon atmosphere. In order to achieve a full homogenization, the samples were melted at least three times. The mass loss of each samples was less than 1.0%. After casting, all samples were wrapped in tantalum foil,

Foundation item: Project supported by the National Basic Research Program of China (2014CB643703), National Key Research and Development Program of China (2016YFB0700901), National Natural Science Foundation of China (51161004, 51371061), Guangxi Natural Science Foundation (2014GXNSFAA118334, 2014GXNSFAA118317, YB1512) * Corresponding author: YAO Qingrong (E-mail: [email protected]; Tel.: +86-773-2291434) DOI: 10.1016/S1002-0721(16)60143-6

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sealed into evacuated quartz tubes for homogenization treatment. The samples were annealed at 1073 K for 20 d, then quenched into an ice-water mixture. The homogenized samples were polished to remove possible oxidized surface before grinding into powder. The ground powders were investigated by X-ray diffraction (XRD) on a PANalytical PIXcel3D power diffractometer with Co Kα radiation, operating at 45 kV and 40 mA and the 2θ scan range of 20º–80º in a step of 0.01313º. Simulation of XRD data was performed using Rietveld crystal structure refinement software (FULLPROF)[17]. Some representative samples were characterized by a HITACHI S-4700 scanning electron microscope (SEM) equipped with energy dispersive spectroscope (EDS). Thermal analysis was carried out by differential scanning calorimetry (DSC, TA-Q600) with a heating rate of 10 K/min from ambient temperature to 1723 K after being isothermal at 323 K for 20 min. High purity N2 was used as purging gas at a constant flowing rate.

2 Results and discussion 2.1 Phase analysis The XRD patterns of (Pr1–xLax)2Fe14B alloys are shown in Fig. 1 for phase identification. It is found that all the intermediate (Pr1–xLax)2Fe14B samples form continuous solid solutions and consist of hard magnetic RE2Fe14B phase (space group P42/mnm) and α-Fe phase (space group Im3m). The structural parameters of all the samples were refined by the Rietveld technique, using the FULLPROF program. Fig. 2 shows the experimental and calculated data of the X-ray powder diffraction patterns of (Pr1–xLax)2Fe14B (x=0 and 0.5), in which the crosses stand for observed intensities and the solid line is the calculated pattern. Vertical lines show reflection positions of the RE2Fe14B and α-Fe. Differences between the observed and the calculated intensities are shown at the bottom of the figure. Based on fitting parameters like smaller values of Rwp, Rexp and χ2, the calculated patterns agree well with the experimental ones. By quantitative

Fig. 1 XRD patterns of (Pr1–xLax)2Fe14B (x=0.0–1.0) samples

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016

Fig. 2 Laboratory X-ray powder diffraction patterns of Pr2Fe14B and (Pr0.5La0.5)2Fe14B recorded at room temperature (black lines) compared with the calculated patterns

phase analyses, RE2Fe14B has been found to be the majority phase, whereas secondary phases (α-Fe) have been identified in much smaller quantities (<2 wt.% in total). It was substantiated by the SEM/EDS microstructure and composition measurement. Fig. 3 shows the SEM of the (Pr1–xLax)2Fe14B (x=0 and 0.5) samples. The grey areas are the Pr2Fe14B or (Pr,La)2Fe14B phase and the dark areas are the α-Fe phase. Compared to Pr2Fe14B, the peaks in XRD pattern are found to shift to lower angle with the increasing amount of La dopant. The variation in the normalized lattice parameters and cell volumes as a function of La concentration (x) in the (Pr1–xLax)2Fe14B series is shown in Fig. 4 and Table 1, from which formation of

Fig. 3 Scanning electron micrographs of (Pr1–xLax)2Fe14B (a) x=0.0, 0.2; (b) x=0.5

YAO Qingrong et al., Crystal structure and phase relations of Pr2Fe14B-La2Fe14B system

Fig. 4 Concentration dependencies of the normalized lattice parameters and unit-cell volumes of the (Pr1–xLax)2Fe14B (x=0.0–1.0) solid solutions (Lines are least-squares fit to the data) Table

x

1

Lattice parameters and cell volumes (Pr1–xLax)2Fe14B (x=0.0–1.0) solid solutions

1123

Fig. 5 Thermogravimetry traces under a low magnetic field for the (Pr1–xLax)2Fe14B

of

Cell volume,

Lattice parameters/nm a

c

c/a

V/nm3

0.0

0.88042 (1)

1.22369 (2)

1.3899

0.948531 (3)

0.1

0.88083 (2)

1.22445 (2)

1.3901

0.950005 (4)

0.2

0.88103 (1)

1.22543 (3)

1.3909

0.950764 (4)

0.3

0.88125 (3)

1.22689 (2)

1.3922

0.952797 (3)

0.4

0.88147 (2)

1.22760 (4)

1.3927

0.953834 (3)

0.5

0.88179 (1)

1.22927 (2)

1.3941

0.955817 (4)

0.6

0.88228 (1)

1.23173 (3)

1.3961

0.958791 (3)

0.7

0.88278 (2)

1.23154 (3)

1.3952

0.960165 (3)

0.8

0.88307 (1)

1.23366 (4)

1.3970

0.962233 (3)

0.9

0.88320 (3)

1.23493 (2)

1.3982

0.963296 (3)

1.0

0.88327 (2)

1.23698 (3)

1.4005

0.965046 (4)

continuous solid solution is clearly proved. The lattice parameter a, c, unit-cell volume V and c/a ratio increase in direct proportion to x. The increase in lattice parameters which may be ascribed to the introduction of large atomic radius of La atoms into Pr2Fe14B lattice would lead to partial substitution of Pr by La, which may be responsible for increase in lattice parameters. 2.2 Thermal analysis and phase diagram Th e Cur ie temp eratu re of th e comp ound s (Pr1–xLax)2Fe14B can be readily determined based on the experiments. The order-disorder magnetic transition gives rise to apparent mass loss of a small piece of sample on the thermogravimetry trace under a low magnetic field exerted by a permanent magnet placed outside the furnace and on top of the crucible position. Fig. 5 shows the Curie temperature of the selected characteristic examples (Pr1–xLax)2Fe14B (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0). The different values of TC corresponding to La contents are represented in Table 2. The results show that 535.7 K for La2Fe14B agrees very well with the 530 K value reported in Ref. [18], and TC (Pr2Fe14B)=559.2 K is slightly lower than that reported by Ref. [19]. As shown in Fig. 6, the Curie temperature (T C ) of (Pr 1–x La x ) 2 Fe 14 B

Fig. 6 Curie temperature (TC) dependence of (Pr1–xLax)2Fe14B compounds and a linear least squares fit to the data

decreases linearly with La content. The increase of the cell volume made Fe–Fe bond distances longer and Fe–Fe interatomic interaction weakened. This resulted in a decrease of the Curie temperature[20,21]. Fig. 7 shows the representative examples of DSC measurement performed on the (Pr1–xLax)2Fe14B (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys. It can be seen that the endothermic peaks appear at 1427.9, 1417.1, 1392.2, 1378.9, 1357.9 and 1348.5 K, respectively, which indicated a peritectic reaction occurring in these samples, liquid+γ-Fe→ (Pr,La)2Fe14B. Thus, the appearance of α-Fe phase in the sample means that the effective cooling rate is not enough

Fig. 7 DSC curves of the (Pr1–xLax)2Fe14B samples

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to prevent a peritectic reaction during solidification[22]. In present work, the DSC results showed no metastable phases in Pr2Fe14B- La2Fe14B system, while the metastable phases existed at Nd2Fe14B–Pr2Fe14B system[16,23]. Other DSC results are listed in Table 2. On the basis of the data from DSC and X-ray diffraction analysis, the phase diagram of the Pr2Fe14B-La2Fe14B system is built up (as seen in Fig. 8). It can be observed that a continuous solid solution covering the whole concentration range exists over a wide temperature range. The temperature of endothermic peaks decreases for all the compositions with an increase of La concentration.

(3) The Curie temperature of (Pr1–xLax)2Fe14B decreased linearly with the increase of La concentration. Based on the results of DSC and XRD, the tentative phase relations of the Pr2Fe14B- La2Fe14B system were constructed. No metastable phases were detected.

Table 2 DSC results measured on heating and data for phase transition TC/

x in (Pr1–xLax)2Fe14B

K

Transition temperature/K Decomposition of (Pr1–xLax)2Fe14B

L+Fe→L

0.0

559.2

1427.9

1567.6

0.1

557.3

1418.6

1560.7

0.2

555.1

1417.1

1563.1

0.3

551.2

1407.6

1542.6

0.4

549.6

1392.2

1535.1

0.5

545.5

1387.6

1522.6

0.6

542.4

1378.9

1518.9

0.7

540.3

1372.2

1505.1

0.8

538.1

1357.9

1500.9

0.9

537.5

1354.2

1496.2

1.0

535.7

1348.5

1492.3

Fig. 8 Phase diagram of the Pr2Fe14B-La2Fe14B system

3 Conclusions (1) X-ray powder diffraction examination revealed that similar to the end-members of the Pr2Fe14B-La2Fe14B formed a continuous solid solution, all intermediate (Pr1–xLax)2Fe14B alloys belonged to tetragonal Nd2Fe14Btype structure (space group P42/mnm). (2) The lattice parameter a, c, unit-cell volume V and c/a ratio of (Pr1–xLax)2Fe14B solid solutions increased linearly with an increase of La concentration.

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