Structure and Phase Transformation in the Giant Magnetostriction

Oct 6, 2017 - ABSTRACT: As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for its abundant magnetic pro...
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Structure and Phase Transformation in the Giant Magnetostriction Laves-Phase SmFe2 Xiaonan Liu,† Kun Lin,*,† Qilong Gao,† He Zhu,† Qiang Li,† Yili Cao,† Zhanning Liu,† Li You,∥ Jun Chen,† Yang Ren,‡ Rongjin Huang,§ Saul H. Lapidus,‡ and Xianran Xing*,† †

Department of Physical Chemistry and ∥State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China ‡ Argonne National Laboratory, X-Ray Science Division, Argonne, Illinois 60439, United States § Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for its abundant magnetic properties. Samarium−iron alloy system SmFe2 is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe2 has been identified as a cubic Fd3̅m structure at room temperature; however, the cubic symmetry, in principle, does not match the spontaneous magnetization along the [111]cubic direction. Here we studied the crystal structure of SmFe2 by high-resolution synchrotron X-ray powder diffraction, X-ray total scattering, and selected-area electron diffraction methods. SmFe2 is found to adopt a centrosymmetric trigonal R3̅m structure at room temperature, which transforms to an orthorhombic Imma structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111]cubic to [110]cubic direction and is further evidenced by the inflection of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the field-cooling−zero field-cooling curve, and the anomaly in the specific heat capacity measurement. The revised structure and phase transformation of SmFe2 could be useful to understand the magnetostriction and related physical properties of other RM2-type pseudocubic Laves-phase intermetallic compounds.

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INTRODUCTION In the past decades, binary intermetallic Laves-phase RFe2 (R is rare earths) has attracted lots of attention owning to their rich magnetic and magnetostriction properties.1−6 Especially, TbFe2 and SmFe2 show huge magnetostriction at room temperature (RT) with saturation magnetostriction coefficients (λs) as large as +1753 and −1560 ppm, respectively, in the polycrystalline forms, which are almost 1 order larger than that of traditional magnetostriction materials.7−10 These large magnetostriction compounds could convert electrical energy to mechanical and have potential applications in devices such as actuators and sensors.1−4 Compared with the TbFe2, SmFe2 displays negative magnetostriction with a much lower magnetocrystalline anisotropy constant K1 (−76 × 10−6 erg/ cm3 for TbFe2 and −5.3 × 10−6 erg/cm3 for SmFe2),11−14 and the magnetostriction behavior of SmFe2 is accessible by a smaller external magnetic field.15−17 The Laves phase SmFe2 has been identified as cubic MgCu2type structure with space group of Fd3̅m (No. 227).18−20 In this model, the rare-earth Sm occupies the 8a sites, and Fe occupies the 16d sites.21,22 It was reported that SmFe2 was ferromagnetic at RT with a magnetostriction constant λ111= −2010 ppm.23,24 However, it is expected that the crystal structure of SmFe2 is likely to distort and deviate from cubic © XXXX American Chemical Society

symmetry along the [111]cubic direction because of the large spontaneous negative magnetostriction under the magnetoelastic interactions.25 In addition, Mossbauer study indicated that the easy magnetization direction (EMD) of SmFe2 at RT orientated along the [111]cubic direction through a rhombohedral distortion, which changed to the [110]cubic direction below 200 K caused by the spin reorientations.26−28 In accompaniment with change of EMD, the orientation of spontaneous magnetic moment turns from [111]cubic direction to [110]cubic direction as well.29,30 All the indications above implied that SmFe2 should adopt a distorted structure other than the cubic symmetry. In this study, we accurately determined the structure and phase transformation of SmFe2 from 100 to 400 K by highresolution synchrotron X-ray powder diffraction (XRD), X-ray total scattering, selected-area electron diffraction (SAED), and magnetic and thermal expansion properties measurements. We show that SmFe2 distorts along the [111]cubic direction from the cubic (Fd3̅m) to be trigonal (R3̅m) at RT and transforms to an orthorhombic phase (Imma) at 200 K. This transition is associated with the conversion of easy magnetization direction Received: October 6, 2017

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DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Structure refinement of the high-resolution SPD pattern of SmFe2 at 300 K. (inset) The real shape of diffraction peaks at the position of (111)cubic and (440)cubic. (b) XPDF structure refinement of the SmFe2 at 300 K with R3̅m structural model. (c) The SAED patterns of SmFe2 at 300 K (up) and the calculated ones using R3̅m model (down) along the [100]R, [110]R, and [111]R directions. Some extra reflections indicated by arrows are caused by double reflections.

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RESULTS AND DISCUSSION According to the phase diagram of the Sm−Fe systems, the melting point of metallic samarium is 1073 K, and the peritectic reaction of SmFe2 compound will occur above 1173 K.31,32 An excess samarium with molar ratio of Fe/Sm = 1.74 should be utilized to achieve the pure phase of SmFe2. Figure S1 (in the Supporting Information) shows the laboratory XRD data of the as-synthesized SmFe2 sample (PDF No. 65−1044), in which no detectable impurities were observed. However, some peaks, such as (111)cubic, (220)cubic, and (311)cubic, display legible shoulders, which do not belong to Fd3̅m and are not caused by the Cu Kα2 radiations. To get further structural information, we performed state-of-art high-resolution synchrotron X-ray powder diffraction (SPD) at the 11-BM-B diffractometer at Argonne National Laboratory from 100 to 400 K using a wavelength of 0.412 63 Å (Figure S2). Figure 1a shows the refinement for the results of SPD data at RT, where the splits of the reflections from cubic symmetry can be clearly observed. For example, the (111)cubic reflection at 5.512° in pseudocubic (Fd3̅m) structure is replaced by coupled peaks at 5.514° and 5.529° in the SPD pattern; the (440)cubic reflection at 18.059° is replaced by peaks at 18.070° and 18.108°. Thus, obviously the actual crystal structure of SmFe2 is not MgCu2 type (Fd3̅m). Indexing of the SPD reflections using DICVOL package resulted in a trigonal lattice with reflection condition rule of (00l: l = 3n; hhl: l = 3n), which is in agreement with trigonal Laves phase of TbFe2 (R3̅m).33 Then, the trigonal R3̅m model was used to refine the structure of SmFe2 at RT, which results in very good agreement between the observed and calculated patterns (Figure 1a, Rwp = 14.2%, χ2 = 2.77). Moreover, the SAED patterns of SmFe2 at 300 K were in agreement with the calculated ones using R3̅m model (Figure 1c). Furthermore, Xray total scattering was used to study the local structure of SmFe2. As shown in Figure 1b, the local structure of assynthesized sample fits well with the measured XPDF using R3̅m model (Figure 1b, Rwp = 6.0%). Figure 2a shows the

and spontaneous magnetization direction. The present structure is helpful to further understand the mechanism of the giant magnetostriction effect in intermetallic compounds.

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EXPERIMENTAL SECTION

The pure Laves compound SmFe2 was prepared by melting of stoichiometric mixture of the constituent elements in a magnetocontrolled arc furnace under a high-purity argon atmosphere. The purities of the components were 99.5% for Sm and 99.99% for Fe. To avoid the presence of the impurities, especially the phases of SmFe3 and Sm2Fe17, an extra 15 atom % Sm was added to compensate for the evaporation loss during melting. To ensure the homogeneity of the ingots, the button samples were turned over and melted several times. The as-cast specimens wrapped in Mo foils were sealed in an evacuated silica tube filled with high-purity argon to prevent oxidation during annealing, homogenized at 1123 K for 5 d, and then quenched by liquid nitrogen. Because the light rare earth element Sm is very active and easily oxidized, the bulk sample was ground into powders in an atmosphere of pure argon to prevent oxygen. The laboratory XRD data were collected at room temperature and low temperature by diffractometer (PANalytical, PW 3040-X’PertPro) with monochromatic Cu Kα radiation. The temperature dependences of high-resolution synchrotron XRD of SmFe2 were collected at the 11-BM-B of the advanced photon source in Argonne National Laboratory with light wavelength of 0.412 63 Å. The total scattering data for X-ray pair distribution function (XPDF) was collected at beamline11-ID-C at the advanced photon source (APS) to recognize the local structure. SAED was performed on a Tecnai G2 F30 STWIN transmission electron microscope with an accelerating voltage of 300 kV. The specific heat capacity data were collected by the physical property measurement system (Quantum Design PPMS-9) in the temperature range of 100−300 K. The precise macroscopic linear thermal expansion curve was measured by an advanced analyzer (NETZSCH DIL402) at the range of 100−350 K with rate of 5 K· min−1. The magnetic measurements were performed with a vibrating sample magnetometer (Quantum Design SQUID-VSM) over a temperature range of 4−400 K. B

DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Atomic Positions and Representative Bond Lengths of SmFe2 (R3̅m) Obtained from SPD at 300 K (up) and XPDF at 300 K (down)

where

θ=

θ1 + θ2 Δθ = θ2 − θ1 2

x

y

z

Sm

2c

Fe1

1b

Fe2

3e

0.1245(0) 0.1249(0) 1/2 1/2 1/2 1/2 Sm−Sm × 1 3.1956(1) 3.2102(0) Sm−Fe2 × 6 3.0752(0) 3.0819(1) Fe2−Fe2 2.6275(0) 2.6309(0)

0.1245(0) 0.1249(0) 1/2 1/2 1/2 1/2

0.1245(0) 0.1249(0) 1/2 1/2 0 0

Sm−Fe2 × 3 3.0790(1) 3.0792(0)

According to eq 1, the magnetostriction coefficient λ111 was calculated based on various-temperature SPD data above 200 K. As shown in Figure 3, the calculated λ111 is −1900 ppm at RT, which is comparable to reported value of −2010 ppm.1 The |λ111| decreases from 3000 ppm at 200 K to 600 ppm at 400 K.

crystal structure of SmFe2 at RT obtained by Rietveld method, where the Sm, Fe1, and Fe2 atoms occupy the 2c (0.1245(0), 0.1245(0), 0.1245(0)), 1b (1/2, 1/2, 1/2), and 3e (1/2, 1/2, 0) sites, respectively. The lattice constants were a = b = c = 5.2441(1) Å and α = β = γ = 60.14(1)°. In this structure, it can be seen that the alternate arrangement of Fe1 and Fe2 atoms follows [101]R direction. We restore the MgCu2-type (Fd3̅m) unit cell in the unit cell of R3̅m (Figure 2b). Similarly with the cubic structure, there is only one Sm site, and the nearest neighbor of an Fe atom is also Fe atoms. In the tetrahedron (composed by Fe atoms), the only Fe1 atom moves following the [111]cubic direction. Figure 2c shows the neighbor atoms of Sm atoms, which is analogous to MgCu2-type (Fd3̅m). It contains 12 Fe atoms that are nearest to Sm atom (Figure 2c). Compared with the simulative “cubic” structure (Fd3̅m), the Sm atoms move ∼0.0062(1) Å along the [111]R direction. The crystallographic [111]cubic direction in pseudocubic structure is equivalent to the [111]R direction in rhombohedral configuration. The distance of Fe1−Fe2 (2.6220(0) Å) is shorter than the Fe2−Fe2 (2.6275(0) Å) distance, which can be seen in Table 1. And the Fe1−Fe1 distance is twice the Fe2−Fe2 distance. It must be mentioned that the nearest neighbor atoms of Sm contain four Sm atoms, which occupy tetrahedron vertex. The distance of nearest neighboring Sm atoms has two values: the Sm−Sm distance paralleling to [111]R direction (3.1956(1) Å) is smaller than other directions (3.2211(5) Å). In the R3̅m structure, the Sm is at the 2c (0.1245(0), 0.1245(0), 0.1245(0)) site and could move along the [111]R direction. This result supports the fact that SmFe2 has a negative magnetostriction along the [111]R of the rhombohedral structure or [111]cubic of the Laves-phase (Figure 2b,c). In analogy with equation of the magnetostriction of the pseudocubic RFe2, the values of λ111 can be described by means of the following equation:21 Δθ tan θ

site

Sm−Sm × 3 3.2211(5) 3.2219(1) Sm−Fe1 × 3 3.0818(0) 3.0849(1) Fe1−Fe2 2.6220(0) 2.6257(0)

Figure 2. (a) The crystal structure of SmFe2 (R3̅m) at 300 K refined from SPD data. (b)The restored model (Fd3̅m) in the unit cell of R3̅m, the dash line stand for Fd3̅m; the solid line represent R3̅m. (c) The interatomic distances in the SmFe12 truncated tetrahedron at 300 K.

λ111 =

atom

Figure 3. Temperature dependences of the magnetostriction coefficient λ111 for the SmFe2 alloy.

When the temperature cools to 100 K, SmFe2 displays a first-order phase transition at 200 K. This phase transition is evidenced in the magnetic and thermal expansion measurement and will be discussed below. Figure 4 shows the SPD data at 100 K; the relative intensity and the position of the reflections changes dramatically compared to that of RT. For example, the two reflections of (101)R at 5.514° and (003)R at 5.529° were replaced by the reflections at 5.5210° and 5.5441° at 100 K; the two reflections, (220)R at 18.0703°and (208)R at 18.1087° for R3̅m structure, split into three reflections at 18.0841°, 18.1360°, and 18.1982° at 100 K. The SPD reflections at 100 K can be well-indexed by the orthorhombic system with cell parameters of a = 5.2185(1) Å, b = 5.2511(2) Å, c = 7.4072(2) Å, and α = β = γ = 90°. According to the high-resolution character of SPD data, the reflection conditions at 100 K follow the rule of (hkl: h + k + l = 2n; hk0: h, k = 2n), which suggests that its space group might be Im2a (No. 46) and Imma (No. 74). The polar Im2a is eliminated because SmFe2 is metallic, and the structure of SmFe2 is solved and

(1)

where θ1 and θ2 represent the interplanar spacing of (220) and (208) reflections, and θ is the average value of θ1 and θ2. C

DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Structure refinement of the high-resolution SPD pattern of SmFe2 at 100 K with Imma structural model. (inset) The real shape of diffraction peaks at the position of (111)cubic and (440)cubic. (b) XPDF structure refinement of the SmFe2 at 160 K with Imma structural model. (c) The SAED patterns of SmFe2 at 110 K (up) and the calculated ones using Imma model (down) along [100]O, [101]O, and [201]O directions.

refined according to the centrosymmetric Imma space group. Figure 4c shows the SAED patterns of SmFe2 at 110 K along [100]O, [101]O, and [201]O directions, which are in agreement with the calculated ones using Imma model. Figure 4 shows the Rietveld refinement pattern against SPD data (see also Table 2); all the observed patterns can be Table 2. Atomic Positions and Bond Lengths of SmFe2 (Imma) Obtained from SPD at 100 K (up) and XPDF at 160 K (down) atom

site

x

y

Sm

4e

Fe1

4a

Fe2

4d

1/2 1/2 0 0 1/4 1/4 Sm−Sm × 2 3.1965(7) 3.2074(4) Sm−Fe2 × 4 3.0738(2) 3.0809(1) Fe2−Fe2 2.6092(0) 2.6210(4)

1/4 1/4 0 0 1/4 1/4

Sm−Sm × 2 3.2160(7) 3.2199(7) Sm−Fe1 × 2 3.0699(7) 3.0729(2) Fe1−Fe1 2.6256(1) 2.6306(1)

z 0.1254(1) 0.1258(2) 0 0 3/4 3/4 Sm−Fe1 × 4 3.0649(2) 3.0760(0) Sm−Fe2 × 2 3.0712(7) 3.0746(2) Fe1−Fe2 2.6181(0) 2.6231(1)

Figure 5. (a) The crystal structure of SmFe2 (Imma) at 100 K from SPD data. (b) The restored model (R3̅m) in the unit cell of Imma, the dash line stands for R3̅m, the solid line represents Imma. (c) The interatomic distances in the SmFe12 truncated tetrahedron at 100 K.

cell of Imma). The local structure of Imma model fits well with the measured X-ray paired distribution function (Figure 4b, collected at 160 K, Rwp = 6.0%). Here, we focus on the structural phase transformation, which has linked to the spin reorientation and rotated EMD.7 The crystallographic [110]cubic direction in Fd3̅m corresponds to the [100]O direction in Imma at 100 K. On the basis of the refinement results, the interatomic distances of nearest neighboring Sm is along the [100]O direction in the context of bond distance analysis; the distance of Fe2−Fe2 along the [100]O direction is also shorter than Fe1−Fe1 along the

perfectly fitted with good fitness (Figure 4a, Rwp = 13.6, χ2 = 2.74). The structural refinement indicated that the splitted SPD reflection stems from the orthorhombic distortion of the cubic RT phase. Figure 5 shows the crystal structure of SmFe2 at 100 K, in which Sm, Fe1, and Fe2 atoms occupied the 4e (1/2, 1/4, 0.1254(1)), 4a (0, 0, 0), and 4d (1/4, 1/4, 3/4) sites, respectively. The sublattice of Fe atoms contains two distinct layers alternately, which are constituted by Fe1 and Fe2, respectively. Sm atoms are inside the caves constituted by Fe sublattice (Figure 5b; we restore the R3̅m model in the unit D

DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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To further study the phase transformation, temperature dependences of the magnetization at an applied field of 1000 Oe of zero-field-cooling (ZFC) and field-cooling (FC) for SmFe2 was measured from 4 to 400 K. As we can see from the FC-ZFC curves in Figure 7a, there is a sharp decline of magnetization at ∼200 K upon heating. In addition, an anomaly also appears in the specific heat capacity at ∼195 K (Figure 7b), which is a powerful proof of the phase transformation. As the SmFe2 sample keeps ferromagnetic over the studied temperature range (Figure S3), the change at 200 K is not ferromagnetic-paramagnetic or antiferromagneticparamagnetic transition but transformation between two ferromagnetic phases (spin reorientation). With the increase of the temperature, the spin reorientation happens accompanying the change of EMD. The characteristic temperature of the phenomenon could be attributed to the direction of the moments and the interaction of Sm and Fe sublattice.

[010]O direction (see Figure 5c). These results were in accordance with the changing of EMD form [111]cubic to [110]cubic direction. This change is also known as spin reorientation. Figure 6a−c shows the temperature dependences of cell parameters refined against various-temperature SPD data.

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CONCLUSION In summary, we have analyzed the crystal structure, magnetic property, and the phase transformation of binary Laves-phase intermetallic compound SmFe2. This study confirmed that SmFe2 possesses a trigonal R3̅m structure at room temperature, which changes to an orthorhombic Imma structure at 200 K. The phase transformation at 200 K is caused by spin reorientation and the change of easy magnetization direction from [111]cubic to [110]cubic. The FC-ZFC curves confirm the fact that the spin reorientation is accompanied by the change of EMD, which is also evidenced by dilatometry measurements and specific heat capacity measurements. Because of the strong absorption of neutrons for the samarium, it is impossible to determine the detailed magnetic structure of SmFe2 by neutron diffractions. Nevertheless, the revised crystal structural information can be helpful to further understand the mechanism of magnetostriction effect and related physical property in Laves-phase as well as other intermetallic compounds.

Figure 6. (a) Temperature dependences of cell edge a-axis and b-axis for SmFe2. (b) Temperatue dependence of cell edge c-axis. (c) Temperature dependences of volume form 100 to 400 K. (d) Dilatometry measurements of SmFe2 from 100 to 350 K.

Considering the phase transformation happens at ∼200 K, we convert the lattice parameter of R3̅m structure by a′ = 2a sin α , b′ = a, c′ = 2

2

(2a cos α2 )

− a 2 , α is the angel between a

and b axes at each temperature. For each crystal structure, it can be seen that the cell parameters increase over the tested temperature range. The axial coefficients of thermal expansion (CTEs) are αa = αb = 10.8(0) × 10−6 K−1; αc = 23.8(0) × 10−6 K−1; αv = 45.6(0) × 10−6 K−1 for the R3̅m structure (200−400 K), and αa = 17.7(0) × 10−6 K−1; αb = 3.9(1) × 10−6 K−1; αc = 7.9(0) × 10−6 K−1; αv = 30.9(1) × 10−6 K−1 for the Imma structure (100−200 K). Figure 6d shows the linear thermal expansion curve measured by dilatometry. A subtle reduction of the bulk volume can be detected at ∼200 K, which corresponds to the first-order phase transition from R3̅m to Imma structure. The linear CTE is αl = 17.0(0) × 10−6 K−1 above 200 K and αl = 12.8(1) × 10−6 K−1 below 200 K, which is comparable to the SPD results (Figure 6d).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02525. Detailed X-ray diffraction data and analysis, temperature dependences of saturation magnetization for SmFe2 (PDF)

Figure 7. (a) Temperature dependences of ZFC and FC magnetization at a magnetic field of 1000 Oe for SmFe2. (b) Temperature dependences of specific heat capacity for SmFe2. E

DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (K.L.) *E-mail: [email protected]. (X.X.) ORCID

Kun Lin: 0000-0003-4515-3206 Jun Chen: 0000-0002-7330-8976 Xianran Xing: 0000-0003-0704-8886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21590793, 21231001, 21701008), National Postdoctoral Program for Innovative Talents (BX201700027), China Postdoctoral Science Foundation (2017M620608), and the Fundamental Research Funds for the Central Universities, China (FRF-TP-17-038A1). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02525 Inorg. Chem. XXXX, XXX, XXX−XXX