Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Negative Thermal Expansion of Ni-Doped MnCoGe at RoomTemperature Magnetic Tuning Qingyong Ren,*,†,‡ Wayne Hutchison,‡ Jianli Wang,*,§,∥ Andrew Studer,⊥ Guohua Wang,† Haidong Zhou,*,†,# Jie Ma,*,† and Stewart J. Campbell‡
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†
School of Physics and Astronomy and Key Laboratory of Artificial Structures and Quantum Control, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China ‡ School of Science, The University of New South Wales at the Australian Defence Force Academy, Canberra, Australian Capital Territory 2600, Australia § College of Physics, Jilin University, Changchun 130012, China ∥ Institute for Superconductivity and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2500, Australia ⊥ Australian Centre for Neutron Scattering, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia # Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: Compounds that exhibit the unique behavior of negative thermal expansion (NTE)the physical property of contraction of the lattice parameters on warmingcan be applied widely in modern technologies. Consequently, the search for and design of an NTE material with operational and controllable qualities at room temperature are important topics in both physics and materials science. In this work, we demonstrate a new route to achieve magnetic manipulation of a giant NTE in (Mn0.95Ni0.05)CoGe via strong magnetostructural (MS) coupling around room temperature (∼275 to ∼345 K). The MS coupling is realized through the weak bonding between the nonmagnetic CoGe-network and the magnetic Mn-sublattice. Application of a magnetic field changes the NTE in (Mn0.95Ni0.05)CoGe significantly: in particular, a change of ΔL/L along the a axis of absolute value 15290(60) × 10−6equivalent to a −31% reduction in NTEis obtained at 295 K in response to a magnetic field of 8 T. KEYWORDS: negative thermal expansion, magneto-structural coupling, neutron powder diffraction, martensitic transformation, Clausius−Clapeyron relation
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INTRODUCTION Because of the special and unusual property of contraction on warming, materials that exhibit negative thermal expansion (NTE) can potentially play important roles in modern technology, finding application in fuel cells, liquid crystal, organic light-emitting diodes, and fiber optics, as well as high precision electronics and optical mirrors.1,2 It has been suggested that NTE is attributable to local structure disorders and/or phase transitions.2−5 Typical changes in local structures involve several processes including the followingtransverse phonon vibration in materials with rigid unit modes or anisotropic thermal vibration (such as ZrW2O8, ScF3, and Zn(CN)4);6−9 local chemical ordering (such as PtNi nanoparticles);10 frame flexibility in metal−organic frames (such as Ag3[Co(CN)6] and CD3OD·D2O);11,12 molecular rearrangement in dumbbell-shaped organic molecule,13 and spin crossover in magnetic complexes (such as [Fe(Rtrz)3]A2· nH2O and (Fe0.84Ni0.16)[Au(CN)2]2).14,15 Meanwhile, the © XXXX American Chemical Society
diverse range of phase transitions that have been shown to induce NTE include spontaneous polarization in ferroelectric PbTiO3,16 magnetovolume effect in Invar alloys, Laves phase alloys and antiperovskites,17−19 electron configuration change in mixed valence materials,20−24 as well as displacive phase transition such as in quartz, ferrierite, and ZrV2O7.25−27 Control or manipulation of NTE in order to realize multifunctional application of materials has risen to importance in recent decades. A variety of effective methods have been employed, such as chemical modification,28,29 introduction of dopants/guests,30,31 nanostructuralization,32 hydriding/hydrating,33,34 as well as the application of pressure.35 In this work, we demonstrate a new route to achieve magnetic manipulation of a giant NTE in (Mn0.95Ni0.05)CoGe via strong magnetoReceived: February 13, 2019 Accepted: April 26, 2019
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DOI: 10.1021/acsami.9b02772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Origin of the NTE in MnCoGe-based compounds. (a) Co−Ge networks in the orthorhombic and hexagonal structures of the (Mn0.95Ni0.05)CoGe sample from different projecting orientations. The crystallographic information is derived from refinement of the neutron powder diffraction (NPD) pattern at 290 K in the zero magnetic field. For easy comparison, different colors and bonding formats are used for the hexagonal and orthorhombic structures. (b) Thermal contour image of the zero-field NPD of (Mn0.95Ni0.05)CoGe over the temperature range from 240 to 320 K. Temperature dependences of (c) lattice parameters and (d) unit cell volumes for both the orthorhombic and hexagonal structures. “Δ” in (c) means changes of the hexagonal lattices relative to the orthorhombic lattices. The inset to (d) shows the variable-temperature massweighted phase fractions for both phases.
properties were studied using magnetization, X-ray and NPD measurements.
structural (MS) coupling around room temperature (∼275 to ∼345 K). Recently, MnCoGe-based compounds were considered as a group of promising NTE materials with giant contraction of the unit cell volume over a reverse martensitic transformation.36−41 It is reported that, with indium doping, the unit cell volume could contract by an amount as large as ΔV/V ≈ 3.9%.36 In MnCoGe-based compounds, there are two stable phasesa low-temperature martensitic phase with the TiNiSitype orthorhombic structure (Pnma) and a high-temperature austenite with the Ni2In-type hexagonal structure (P63/ mmc).42 Both of the structures are built on the Co−Ge network, as shown in Figure 1a.43,44 The lattice parameters of the hexagonal and orthorhombic structures are related by: aorth ≈ chex, borth ≈ ahex, corth/√3 ≈ chex, and Vorth/2 ≈ Vhex. In the parent MnCoGe sample, there is a displacive martensitic transition from the hexagonal structure to the orthorhombic structure at TM ≈ 650 K, which can be lowered readily, for example, through selective doping.45,46 In addition, the orthorhombic and hexagonal phases exhibit ferromagnetic 42 hex If ordering at Torth C ≈ 345 K and TC ≈ 275 K, respectively. TM is adjusted in a designed manner into the temperature range of 275 to ∼345 K, it is possible to obtain an MS coupling47 and thereby realize a magnetic manipulation on the NTE. In the present work, an MS coupling was designed in MnCoGe through partial substitution of Mn with Ni. The magnetic responses of the microstructures and of the NTE
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EXPERIMENTAL SECTION
Sample Preparation. The (Mn0.95Ni0.05)CoGe samples were prepared by arc melting stoichiometric amounts of Mn, Ni, Co, and Ge under an argon atmosphere. The surface of the Mn flake was removed using diluted nitric acid. Each sample was melted four times to ensure homogeneity. The obtained ingots were wrapped with tantalum foil and then sealed into quartz tubes with vacuum. The samples were annealed at 850 °C for 7 days and then quenched into ice water. In order to reduce the potential effects of internal stress,40,48 powder samples were obtained by grinding the ingot (mortar and pestle) for a relatively short time of 5 min. X-ray Diffraction Measurements. The sample qualities were checked with room-temperature X-ray diffraction (XRD) measurement. The variable-temperature XRD measurement was carried out using the PANalytical diffractometer with Cu Kα radiation over the temperature range of 20−310 K. Physical Property Measurements. Temperature-dependent magnetization measurement was performed on a Quantum Design physical property measurement system (PPMS). A continuous reading mode was used with a heating rate of 3 K/min over the temperature range of 2−345 K. NPD Measurements. NPD measurements were performed on the (Mn0.95Ni0.05)CoGe sample using the high-intensity powder diffractometer, WOMBAT (λ = 2.4205 Å), at the OPAL Reactor (Lucas Heights, Australia). Vanadium holders were used and the powder sample was fixed with aluminum foil. The zero-field measurements were carried out from 5 to 320 K. For the measurement with magnetic fields, the sample was cooled down to 220 K under zero field, then the B
DOI: 10.1021/acsami.9b02772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces magnetic field was applied, and data were collected from 240 to 320 K, with a data acquisition time of 5 min after waiting 1 min for thermal equilibrium. The magnetic structure was determined using irreducible representation based on the BasIreps software and Rietveld refinements of both the NPD and XRD patterns were carried out using the FullProf suite.49 The errors for the lattice parameters and the corresponding NTE values were estimated from Rietveld refinements of the NPD data on multiplication of the Rietveld refinement uncertainties and the standard deviation. The ΔL/L values were calculated as (L − L0)/L0, where L0 is the reference temperature (taken as 270 K in this work). As discussed in the next section, the three-dimensional distribution of the lattice thermal expansion coefficient was illustrated using the PASCal program based on the temperature-variable lattice parameters determined from NPD.50
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RESULTS AND DISCUSSIONS NTE without Magnetic Field. A thermal contour image of the zero-field NPD for (Mn0.95Ni0.05)CoGe is shown in Figure 1b. The martensitic transformation temperature, TM, decreases from ∼650 K (MnCoGe) to 290(1) K for (Mn0.95Ni0.05)CoGe by Ni-doping on the Mn-site. The crystallographic and magnetic structures are extracted from the NPD patterns through Rietveld refinement (see Figures S1 and S4). As shown in Figure 1c, ahex exhibits a tiny change relative to corth/ √3 but a pronounced expansion of 6.93% relative to borth. On the other hand, a giant contraction of −10.19% was observed for aorth → chex. This giant contraction along aorth causes an overall contraction for the unit cell volume, with ΔV/V (Vorth/ 2 → Vhex) amounting to −3.74%. This value is similar to the values of −3.9% attained for MnCoGe1−xInx (x = 0.01, 0.02) and −3.8% for MnCoGe0.97Sb0.03.36 In order to track the NTE across the reverse martensitic transformation, the average lattice parameters and average unit cell volumes (calculated from Lave = Lorth × forth + Lhex × f hex where f is the corresponding phase fraction) are considered in the following discussion (see Figure 1d as an example). The changes of the average lattice parameters, aave, bave, cave, and the overall unit cell volume, Vave, as a function of temperature across the martensitic transition are shown in Figure 2a. The aave-axis shrinks by ΔL/L ≈ −80180(60) × 10−6 over a temperature window of 50 K around room temperature (from 270 to 320 K), and the overall unit cell volume, Vave, contracts by a value of ΔV/V ≈ −26 000(40) × 10−6. The spatial distribution of the thermal expansion coefficients for (Mn0.95Ni0.05)CoGe under zero magnetic field is calculated using the PASCal program (a linear approximation is used here).50 As shown in Figure 2b, (Mn0.95Ni0.05)CoGe exhibits pronounced anisotropy in the martensitic transformation-associated thermal expansion. The thermal expansion coefficients, αL, are −1804(105) × 10−6, 1265(71) × 10−6, and 46(2) × 10−6 K−1 along the aave, bave, and cave axes, respectively. The overwhelming contraction along the aave leads to a giant overall NTE coefficient (defined as αL = dV/ V dT) of αV ≈ −621(35) × 10−6 K−1, which is significantly larger than bench mark NTE materials such as ScF3 (−14 × 10−6 K−1)51 and in ZrW2O8 (−27.3 × 10−6 K−1).6 NTE under Magnetic Fields. The effects of applied magnetic fields on the NTE of (Mn0.95Ni0.05)CoGe were studied by the NPD measurements under magnetic fields of 0, 2, 5, and 8 T. According to the phase fraction evolution with temperature (Figure S2), the martensitic transition temperature, TM, moves toward higher temperature with increasing field. Giant changes in the NTE properties of (Mn0.95Ni0.05)-
Figure 2. Zero-field NTE in (Mn0.95Ni0.05)CoGe. (a) Changes in the average lattice parameters, aave, bave, cave, and the overall unit cell volume, Vave. (b) Expansivity indicatrix of (Mn0.95Ni0.05)CoGe under zero field, determined using the PASCal program (a linear approximation is used here),50 showing the spatial orientation of strong positive (red) and negative (blue) thermal expansion. A linear approximation over the temperature range from 270 to 320 K was used in the determination of the expansivity indicatrix.
CoGe are observed with the application of magnetic field. As illustrated in Figure 3, the NTE of both aave and Vave are suppressed dramatically by the external field. For example, ΔL/L(aave) was changed by an absolute value of 15 290(60) × 10−6 from −48 974(40) × 10−6 (0 T) to −33 680(20) × 10−6 (8 T), and ΔV/V was changed by 4456(50) × 10−6 from −16 430(30) × 10−6 (0 T) to −11 970(20) × 10−6 (8 T) at 295 K. The ratios of these relative changes, [ΔL/L(8 T)]/[ΔL/L(0 T)] and [ΔV/V(8 T)]/[ΔV/V(0 T)], attain values of −31 and −27%, respectively. This compares, for example, with a ∼−36% reduction (5 T) in NTE for the CdCr2S4, which has been considered as a relaxor ferroelectric.52,53 The resulting thermal expansion coefficients (in the range 270−320 K and assumed linear) are summarized in Table 1. Both |αL| and |αV| are subjected to considerable reductions in value between zero field and 8 T, such as Δ|αL(aave)| ≈ 80 × 10−6 K−1 and Δ|αV| ≈ 13 × 10−6 K−1. Microscopic Responses to Magnetic Field. To investigate the response of the NTE of (Mn0.95Ni0.05)CoGe to magnetic fields, the temperature dependence of the zerofield-cooled magnetization (similar to the NPD measurement protocol) was measured under a series of fields from 0.01 to 9 T over the temperature range of 200−345 K (Figure 4a, full set of data shown in Figure S3). The magnetic transition temperatures, TC, were determined from the derivative of the magnetization curves (Figure 4b). For the data of Figure 4c, the TM values (as determined from NPD) were found to C
DOI: 10.1021/acsami.9b02772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
ment at a critical point, rather a concomitant phenomenon of the structural transition between the ferromagnetic-orthorhombic structure and the paramagnetic-hexagonal structure. This response is similar to the behavior exhibited by other magnetocaloric materials with MS coupling (e.g., Gd5Si2Ge2).57 The magnetic moments, determined from Rietveld refinement, demonstrate an apparent increase with magnetic field from 0 to 2 T, and then reach saturation as shown in Figure 4e; hence, TC increases more rapidly than TM with magnetic field in the range of 0−2 T (see Figure 4c). Our previous study indicated that each atom in the crystal cell is displaced by a small amount during the transition from the hexagonal structure to the orthorhombic structure, and that the three-dimensional five-connected (3D5C) Co−Ge network, which builds on the edge-sharing six-membered Co− Ge rings, switches to a 3D4C network (also see Figure 1a).44 In the orthorhombic structure, the interstitial Mn atoms penetrate through the 3D4C network and form a threedimensional network as depicted in Figure 4d (and Figure S4).58 Crystal orbital overlap population calculations on the TiNiSi-type orthorhombic structure demonstrated that there exist weak but nontrivial bonding between Ti and Si atoms which correspond to the Mn and Ge atoms in our case.43 This bonding establishes an interaction between the Mn−Mn and Co−Ge networks. Although the Co−Ge network is the dominant influence on stability of the structures,43,58 it is still possible to influence the overall structure by tuning the Mn−Mn network. The Mn−Mn distance, d 1 , in the orthorhombic structure contracts with magnetic field from 3.114(1) Å (0 T) to 3.074(1) Å (8 T), while d2 expands from 3.200(1) Å (0 T) to 3.295(1) Å (8 T) (see Figure 4f). The changes in the Mn−Mn distance correspondingly affects the shape of the six-membered Co−Ge rings, which evolve from distortion of the flat counterpart in the hexagonal structure. The distortion angles α and β (marked in Figure 4d) were suppressed as shown in Figure 4g, ultimately leading an obvious contraction of the aorth axis (see Figure S5 in Supporting Information). Similar reshaping of the sixmembered rings due to changes in d1 and d2 were also observed in MnNiGe when the Mn was replaced by Fe.43 Another interesting phenomenon is that the influence on the lattice parameters by the magnetic field mainly occurs around the MS transition. As shown in Figure 5a, the aorth axis only exhibits an obvious change with magnetic field above 280 K. This phenomenon can be explained by the effect of the magnetic field on the martensitic transformation temperature, TM, which is described by the Clausius−Clapeyron relation59,60
Figure 3. NTE in (Mn0.95Ni0.05)CoGe for different magnetic fields. Relative changes in (a) aorth axis, ΔL/L, and (b) unit cell volume, ΔV/V. The insets show the field dependences of the ΔL/L and ΔV/V values at 295 K and the corresponding percentage changes with respect to the value obtained in 0 T magnetic field.
Table 1. Thermal Expansion Coefficients along the Three Unit Cell Axes, and the Overall Unit Cell Volume in (Mn0.95Ni0.05)CoGe over the Temperature Range of 270− 320 K in Different Magnetic Fields μ0H (T)
αL-aave (×10−6 K−1)
αL-bave (×10−6 K−1)
αL-cave (×10−6 K−1)
αV (×10−6 K−1)
0 2 5 8
−1804(105) −1757(82) −1745(62) −1724(80)
1265(71) 1218(55) 1223(43) 1203(55)
46(2) 42(2) 41(2) 36(3)
−621(35) −608(29) −587(23) −592(31)
exhibit a linear dependence on the external magnetic field (dT/d(μ0H) = 0.99(7) K T−1) with TC exhibiting a similar linear dependence above 2 T (dT/d(μ0H) = 0.93(2) K T−1). These values are comparable to dT/d(μ0H) = 0.9 and 1.11(4) K T−1 as determined from heat capacity measurements for MnCo0.98Cu0.02GeB0.02 and (Mn0.97Ni0.03)CoGe, respectively.54,55 The small difference (∼3 K) between TC and TM over the field range 2−8 T, is considered to stem from the different heating rates in the magnetization and NPD measurements. In addition, Rietveld refinements of the NPD patterns around TC (TM) demonstrate an evolution from the ferromagnetic orthorhombic structure to the paramagnetic hexagonal structure (see Figure S1 in the Supporting Information), indicating the formation of strong MS coupling in (Mn0.95Ni0.05)CoGe.46,47,56 The simultaneous variation of TC and TM is a direct result of the shift of the MS transition temperature. The magnetic transition in this case is not a conventional spontaneous-polarization of the magnetic mo-
μ0 dH dT
=
ij ΔM yz ΔSM zΔH and ΔT = μ0 jjj j ΔS zzz ΔM k M{
(1)
where ΔM and ΔSM are the differences in magnetic moments and entropy between the martensitic and austenitic phases and ΔT is the shift in transition temperature resulting from ΔH, the change in magnetic field. In order to obtain a large magnetic influence on the structural stability or a large value of ΔT, a large ΔM but small ΔSM are required. In MnCoGe-based compounds, the ferromagnetic ordering temperatures of the orthorhombic and the hexagonal phases hex are below the Curie temperatures, Torth C ≈ 345 K and TC ≈ 42 275 K, of the pure MnCoGe compound. In addition, the saturated magnetization of the orthorhombic phase is generally larger than that of the hexagonal phase as illustrated in Figure D
DOI: 10.1021/acsami.9b02772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Effects of magnetic field on the microscopic structures of (Mn0.95Ni0.05)CoGe. (a) Iso-field magnetization and (b) corresponding derivative under different magnetic fields. (c) Field dependences of the martensitic transformation temperature, TM, determined from NPD and of the magnetic transition temperature, TC, determined from magnetization measurements. (d) Top panel: schematic diagram of the structural evolution of the orthorhombic structure with application of magnetic field. Each Mn atom has four Mn nearest-neighbors with two distinct groups of different Mn−Mn distances, marked as d1 and d2. Bottom panel: schematic diagram of the distortion of the Co−Ge six-membered ring with the angles of α and β. (e) Temperature dependence of the magnetic moments in (Mn0.95Ni0.05)CoGe under different magnetic fields. Magnetic field dependence of the (f) Mn−Mn distances and (g) distortion angles, α and β, of the six-membered ring at 295 K.
Figure 5. Enhanced effects of magnetic field on NTE through MS coupling. (a) Change in length of aorth for (Mn0.95Ni0.05)CoGe with magnetic field at different temperatures (for raw data, see Figure S7). (b) Schematic figure for the magnetic-field-tunable NTE in MnCoGe-based compounds. The two gray lines represent the magnetization of the hexagonal and orthorhombic phases of the pure MnCoGe compounds. Given the synchronous changes in the magnetization and lattice parameters among the MS coupling temperature window (see Figure 4c), the blue lines represent the schematic behavior of both magnetization and ΔL/L. The left and right insets in (b) are the structures on either side of the transition; the magnetic structure of the orthorhombic structure in (Mn0.95Ni0.05)CoGe at 5 K and the (paramagnetic) crystal structure of the hexagonal structure at 320 K, determined from the zero-field NPD.
Table 2. NTE Coefficient, α, operating Temperature Range, NTE Mechanism, and Magnetic Effect on NTE in Some Typical NTE Materials Tuned by Magnetic Fielda materials (Hf,Ta)Fe2 EuTiO3 BiOCuSe CdCr2S4 Bi2Sr2−xLaxCuO6 La2−xSrxCuO4 κ-(BEDT-TTF)2Cu(NCS)2 (Mn0.95Ni0.05)CoGe
α (×10−6 K−1) −123 −4
∼−10 ∼−3.5 ∼−2.3 −621(35)
temperature range (K)
NTE mechanisms
magnetic effect on NTE
refs
∼220 to ∼225
