Enhancing Magnetic Functionality with Scandium: Breaking

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Enhancing Magnetic Functionality with Scandium: Breaking Stereotypes in the Design of Rare Earth Materials Yaroslav Mudryk, Durga Paudyal, Jing Liu, and Vitalij K. Pecharsky Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00314 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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Chemistry of Materials

Enhancing Magnetic Functionality with Scandium: Breaking Stereotypes in the Design of Rare Earth Materials Yaroslav Mudryk,1* Durga Paudyal,1 Jing Liu,1,2,3 Vitalij K. Pecharsky,1,2 1

Division of Materials Science and Engineering, Ames Laboratory of US DOE, Iowa State University, Ames, Iowa, 50011-3020 2

Department of Materials Science and Engineering, Iowa State University,

Ames, Iowa, 50011-2300 3

Current Affiliation: Seagate Technology, Bloomington, MN, USA

ABSTRACT: Replacement of strongly magnetic gadolinium with weakly magnetic scandium unexpectedly enhances ferromagnetic interactions in (Gd1-xScx)5Ge4. Based upon this counterintuitive experimental finding we demonstrate the unique role 3d1 electrons of scandium atoms play in mediating magnetic interactions between the gadolinium atoms from the neighboring layers in the Sm5Ge4-type crystal lattice. Scandium substitutions at and below 20% rapidly increase the Curie temperature, TC, of the Gd5Ge4 parent, eliminate both the kinetic arrest and hysteresis, and drastically improve reversibility of the first-order magnetostructural transformation at TC. In agreement with first principles predictions, higher than 20% Sc lead to a formation of a closely related Pu5Rh4-type structure where the first-order magnetostructural transformation is replaced by a conventional second-order ferromagnetic ordering that remains accompanied with a secondorder rearrangement of the crystal lattice. Comparison of two materials with similar structures and compositions shows that significantly stronger magnetocaloric effect occurs in the first-order material, which also shows very small hysteresis. Furthermore, we demonstrate that a behavior of a specific interatomic distance can predict anomalous physical properties in a series of alloys where compositional dependence of lattice parameters suggests a rather trivial solid solubility and uninteresting magnetism.

1. Introduction Targeted discovery of new materials, developing efficient syntheses and controls to realize specific physical properties dovetails modern chemistry with physics, and experiments with first-principles calculations and informatics.1,2 As it has been for decades, expanding the boundaries of knowledge implies avoiding stereotypes that may constrain scientific exploration, e.g., due to perceived lack of interesting science or due to a high cost of doing research. Take Scandium, for example: it is one of the elements rarely mentioned in connection to either technological applications or basic science. Chemically, it has a dual nature: while Sc fits within the rare earth family of the elements that also includes Y and lanthanides (La-Lu), it is also the very first d-element in the Periodic Table. Scandium metal exhibits a number of useful properties, such as excellent electrical conductivity, creep resistance, and low density. Unlike the majority of rare earths (lanthanides represent 75% of all known magnetic elements), Sc is perceived as essentially non-magnetic element regardless of the metal itself being a Curie-Weiss paramagnet with an effective magnetic moment of 1.6 µB/atom.3 High price and uncertain supply effectively prevent its widespread use, yet scandium-containing materials have found high-end applications ranging from

solid oxide fuel cells to sports equipment,4,5 but at present they have little to no use in magnetism. However, our work shows that scandium containing magnetic alloys have a potential to become novel functional materials. Scandium-containing compounds with relatively high magnetic ordering temperatures are known, for example SmScGe6 and GdScGe7 order ferromagnetically at Curie temperatures, TC, of 270 K and 350 K, respectively, but the high TCs can be attributed to the presence of magnetic lanthanides (Sm and Gd) and peculiar crystal structures that support strong ferromagnetic exchange. For example, GdScGe crystallizes in the CeFeSi-type structure and has a high TC, while GdScSb, which adopts a related CeScSi-type structure, orders antifferomagnetically at TN = 56 K.8 Following conventional wisdom, imagining a system where magnetism is enhanced solely by the presence of Sc is counterintuitive, and much more so due to a replacement of a strongly magnetic element like Gd with Sc. At the same time, it would be interesting to see how such substitutions may play out in the R5T4 systems of intermetallic compounds (R – rare earth element, T – group 13-15 elements, such as Si, Ge, Sn, Sb, Ga etc.), where the substitution of one non-magnetic T element by another can drastically change the magnetic behavior.9,10,11

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The R5T4 compounds exhibit a plethora of interesting and technologically relevant physical phenomena, including (but not limited to) giant magnetocaloric effect,12,13,14 giant colossal magnetoresistance,17-20 magnetostriction,15,16 21 spontaneous generation of voltage, kinetic arrest,22,23 and short-range magnetic correlations.24,25 Most of them are rooted in the magnetostructural transformations that are sensitive to the chemical composition of the alloy and occur as temperature, pressure, and/or magnetic field are altered.26 The crystal structures of the R5T4 compounds may be conveniently represented as sequences of sub-nanometerthick two-dimensional layers (slabs) that are stacked along the long axis (b-axis) of the unit cell (Figure 1). The slabs can shift laterally with respect to each other in response to external stimuli resulting in a change of crystal structure and, consequently, rich magnetism.27 The intimate coupling of magnetic and crystallographic lattices in R5T4 systems is often manifested by a wellestablished relationship between the presence/absence of defining interatomic bonds (typically these are specific TT bonds between the slabs) and the magnetic state of the compound. For example, when the T-T bonds are short (strong), this establishes the O-I Gd5Si4-type structure which is nearly always associated with ferromagnetism (FM), but when the bonds are much longer (weaker) in the O-II Sm5Ge4-type structure, its ground state becomes antiferromagnetic (AFM). These two structures, O-II and O-I, can directly transform into each other during coupled magnetostructural phase transitions. Recent investigations of the effect of chemical substitutions on the electronic structure and magnetism of Gd5Ge4 compound highlighted several important facts. First, the magnetic moments within the slabs order ferromagnetically28 but it is the communications (exchange interactions) between the slabs that determine what kind of magnetic state (FM, AFM, or short-range magnetic ordering) is present in the bulk at given thermodynamic conditions.29 Second, while the type of the crystal structure and the interslab T-T distance has the major impact on the magnetism, it is also possible to fine-tune the magnetism of Gd5Ge4 by using targeted substitutions of specific Gd crystallographic sites by other rare earths taking advantage of lanthanide contraction.30 Third, it was found that changes in the electronic structure associated with chemical substitutions dominate the size effects, such as chemical pressure.31 Namely, the rapid increase of TC with x in the Gd5(SixGe1-x)4 system is mainly caused by the stronger Si 3p – Gd 5d hybridization compared to the Ge 4p – Gd 5d hybridization. Our previous work also showed that the Gd/R substitutions targeting the Gd atoms located inside the slabs (intraslab Gd atoms) typically have much greater effect on the system compared to other lattice positions occupied by Gd. Since this intraslab position shows affinity for smaller rare earth atoms, the Sc atoms are anticipated to replace Gd there first. Because the location of this position tends to naturally en-

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hance the influence of the substituting atom, we expect to see a strong effect on the magnetic properties. If Sc remains non-magnetic, the substitution would most likely prevent the establishment of the FM state in (Gd1-xScx)5Ge4, as does Lutetium in very small (2.5-5 at.% overall) concentrations.30 Investigation of magnetic properties of Gd2Sc3Ge4 [(Gd0.4Sc0.6)5Ge4] indicates AFM state below TN = 22 K.32 However, one may also imagine that at low x(Sc) the potentially strong Sc 3d – Gd 5d hybridization may complement chemical pressure31 and lead to the enhancement of ferromagnetism in (Gd1-xScx)5Ge4. Here we report a systematic investigation of the (Gd1-xScx)5Ge4 system by experimental (temperature and magnetic field dependent Xray powder diffraction and magnetic measurements) and theoretical (density functional theory, DFT, calculations) methods. 2. Experiment and Calculation Details Experimental: The (Gd1-xScx)5Ge4 samples with x = 0.025, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.75, and 1 with 5 g total mass each were prepared by arc-melting of the pure elements (the rare earth metals were prepared by the Materials Preparation Center of the Ames Laboratory33 and were 99.95+ wt.% pure with respect to all other elements in the Periodic Table) in an argon atmosphere. Phase content and room temperature crystal structure of the samples were analyzed using the X-ray powder diffraction (XPD) patterns obtained on a Philips X’Pert Pro diffractometer (Cu Kα1 radiation). Temperature-dependent XPD experiments were performed in zero and applied magnetic fields using a Rigaku TTRAX X-ray powder diffractometer (Mo Kα radiation) equipped with a continuous helium-flow cryostat and a superconducting magnet.34 Rietveld analysis of the XPD data was performed using LHPM Rietica35 and FullProf.36 Pseudo-Voigt function was used to model the peak shapes; relevant profile parameters were refined for each composition and structure but fixed for the minority phases in the mixed-phase regions during the temperature and field dependent measurements. All atomic coordinates except the symmetryrestricted ones were refined. For sites statistically occupied by Gd and Sc atoms the atomic coordinates of Gd and Sc were constrained to be identical and site occupations were constrained as follows: x(Gd) + x(Sc) = 1. Except for the specific cases presented in Tables S1-S4 (FullProf refinements), all atoms were assumed to have identical displacement parameters. For refinements using Rietica the background was treated using the standard polynomial function; however, for FullProf the refinement using pre-set background points was found to be a better option. The dc magnetization was measured in a Quantum Design MPMS-XL7 magnetometer using bulk polycrystalline pieces. Calculation: The local spin density approximation including onsite 4f electron correlation37 and spin orbit coupling (LSDA+U+SOC) approach has been employed to

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understand magnetostructural behaviours of (Gd0.8Sc0.2)5Ge4. The LSDA+U+SOC is implemented in the tight binding linear muffin tin orbital (TB-LMTO)38 and full potential linear augmented plane wave (FP-LAPW)39 methods. The orbital dependent Coulomb and exchange interactions in LSDA+U shift occupied 4f states towards the lower energy and unoccupied 4f states towards the higher energy. In Gd systems the orbital angular momentum quantum number L is zero that gives total angular quantum number, J = spin angular quantum number, S = 7/2. The small but non negligible 5d orbital moment contributes to the spin orbit coupling leading to a weak but non negligible magnetocrystalline anisotropy.40,41 The electronic structure calculations performed with different values of Hubbard U (Coulomb repulsion between onsite 4f electrons) ranging from 1 to 7 eV indicate that with the higher values of the U, the occupied 4f states are shifted to the lower energy while the unoccupied 4f states are shifted to the higher energy, as expected. Experimental values of U = 6.7 eV and J (exchange interaction between onsite 4f electrons) = 0.7 eV reported for Gd metal37 have been used in these calculations. The k-space integrations have been performed with 16 × 16 × 16 Brillouin zone mesh, which was sufficient for the convergence of total energies and magnetic moments. Additionally, we have performed structure relaxation for (Gd0.8Sc0.2)5Ge4 placing Sc in the 4c sites. Calculations with the relaxed parameters lead to the same conclusions as when using experimental crystal parameters.

affect the magnetic state of Gd5Ge4 much despite the fact that La is non-magnetic as both Lu and Y are. The formation energies with Sc substitutions in Gd2 and Gd3 sites are also negative but they are nearly 1 eV/cell higher compared to the Sc substitution in Gd1 site, clearly indicating that at low x, Sc is most likely to replace Gd1 in (Gd1-xScx)5Ge4. According to the calculations, the Pu5Rh4-type and not the O(II)-Sm5Ge4 was found to be the most stable crystal structure for the (Gd0.8Sc0.2)5Ge4 (x = 0.2, or 20% total replacement of Gd by Sc). The total energy of the Pu5Rh4 type orthorhombic structure is lower by 3.422 eV/cell compared to the Sm5Ge4 type (O-II). Surprisingly, calculated results showed ferromagnetic ground state in Pu5Rh4-type (Gd0.8Sc0.2)5Ge4 (with the b-axis being the easy magnetization direction) contrary to the antiferromagnetic ground state, both observed and calculated, with other (Lu, La, or Y) substitutions and in contrast to the AFM state reported for (Gd0.4Sc0.6)5Ge4.32 These unexpected theoretical results motivated a thorough experimental investigation of the (Gd1-xScx)5Ge4 system.

3. Results Both Gd5Ge4 and Sc5Ge4 crystallize in the same crystal structure, O(II)-Sm5Ge4, and considering ~9% difference of metallic radii of Gd (1.80 Å) and Sc (1.64 Å), extensive solid solubility of Sc in Gd5Ge4 may be expected. Continuous solid solubility, on the other hand, is unlikely because (Gd0.4Sc0.6)5Ge4 adopts either the Pu5Rh4-type structure42 or forms as an ordered superstructure of Ce2Sc3Si4 type,32 where one 8d position is occupied exclusively by Gd and two other positions, 4c and 8d, reveal 100% Sc occupation. The preferential Sc substitution for each particular Gd site was estimated theoretically by calculating formation energies at 0 K with several different scenarios (there are three unique rare earths sites in the structure) using the equation: ∆E = EGd5-xScxGe4 – (5-x)EGd – 4EGe - xESc, where E and x represent total energies and molar concentrations, respectively. Sc atoms were placed in the individual positions of the Gd atoms in the known Sm5Ge4 type (O-II), Gd5Si4 type (O-I) (structural parameters taken from Ref. 43), and Pu5Rh4 type42,44 orthorhombic structures of Gd5Ge4. The lowest (most negative) formation energy was found when Sc replaces Gd1 (4c), in agreement with the previous theoretical and experimental results obtained for the Lu and Y substitutions, where the substitution of Gd1 atoms by smaller non-magnetic rareearths also leads to a substantial reduction of itinerant (delectron) magnetism.30,45 The same study30 showed that larger La atoms prefer Gd2 and Gd3 (8d) sites and do not

Figure 1. Crystal structure types observed in the (Gd1-xScx)5Ge4 pseudobinary system. Large light-blue spheres represent intraslab (4c) Gd1 (Sc1) sites, large dark-blue spheres are interslab (8d) Gd2 and Gd3 (Sc2 and Sc3) sites; intraslab Ge1 and Ge2 sites are shown as small yellow spheres, while the interslab Ge3 atoms are small red spheres. The Ge3-Ge3 bonds are marked as solid, dashed, or broken lines depending on bond lengths indicative to the type of crystal structure.

The prepared (Gd1-xScx)5Ge4 samples were single phase alloys except for the binary Sc5Ge4 (which was ~85 mol.% pure, with Sc5Ge3 and Sc11Ge10 impurity phases being consistently present, prolong annealing at 700 and 1000 °C increases amount of secondary phases) confirming that scandium readily replaces gadolinium through the whole range of compositions. As shown in Figure 2, lattice dimensions of (Gd1-xScx)5Ge4 alloys gradually decrease with x as the smaller Sc compresses the lattice. Interestingly, the compression is anisotropic and non-linear with the negative deviation along the a-axis and the positive deviation along the c-axis. The anisotropy is most pronounced in the middle but it disappears at the end (at x = 1). Such unusual change suggests that (Gd1-xScx)5Ge4 alloys experience the effect of anisotropic chemical pressure magnified

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by the preferential substitution of two rare earth atomic positions (especially, the intraslab 4c site) over the third one, in agreement with the published crystal structure of (Gd0.4Sc0.6)5Ge4.32,42 This third rare earth site, located on the edge of a slab, does not accept scandium until more than 30% of Gd is substituted by Sc. Such uneven Gd/Sc occupation must create a substantial distortion of atomic environment both within the otherwise rigid slabs and between them. From the examination of the lattice parameters (Figure 2a) and the unit cell volume (Figure 2b) there is no clear indication of a composition-induced structural transition in the (Gd1-xScx)5Ge4 series. At the same time, there are several sharp changes in the characteristic interslab Ge3Ge3 distance (dGe3-Ge3), the typical indicator of a structural change in R5T4 compounds,9 which are in contrast with the smooth decrease of the unit cell volume. In particular, for x=0.3 the dGe3-Ge3 value drops sharply indicating a shift into the Pu5Rh4-type structure, where the dGe3-Ge3 distance (3.1 Å) is shorter than in the Sm5Ge4-type (3.6 Å) but is longer compared to the dGe3-Ge3 bond distance in the Gd5Si4-type (2.7 Å) (Figure 1). This drop coincides with the change from mostly isotropic to strongly anisotropic lattice contraction. Furthermore, dGe3-Ge3 ceases to decrease when the Sc concentration reaches 50%, and the binary Sc5Ge4 has a much longer dGe3-Ge3 compared to the (Gd0.25Sc0.75)5Ge4 sample. This increase in dGe3-Ge3 is also coupled with the change from the anisotropic contraction of the lattice parameters to the nearly isotropic one. Thus, we conclude that the (Gd1-xScx)5Ge4 alloys in the middle of the diagram belong to the Pu5Rh4-type structure, while at the ends of the pseudobinary diagram the structure type is Sm5Ge4.

Figure 2. a) Anisotropic contraction of the (Gd1-xScx)5Ge4 lattice as a function of Sc concentration; b) comparison of the gradual decrease of the unit-cell volume with the anomalous change of interslab Ge3-Ge3 distance. Vertical dashed lines mark the approximate phase boundaries between Sm5Ge4 and Pu5Rh4 structures.

As will be shown below, these deviations from linearity in dGe3-Ge3(x) dependence at room temperature correspond to the changes in low-temperature magnetostructural behaviour. This is a very interesting and important observation, because it shows that a composition-induced change of a particular structural parameter (in our case, a specific

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interatomic distance dGe3-Ge3) at room temperature may be used to predict low-temperature anomalies even when the compositional dependence of lattice parameters suggests a simple, although non-ideal, solid solution. Further, we performed the analysis of the low-temperature O(I)-Gd5Si4 structure for x = 0, 0.05, 0.2, and 0.3, which reveals that a smooth decrease of lattice parameters with x(Sc) coexists with a rapid increase in the dGe3-Ge3 (Figure S4). The increase in dGe3-Ge3 agrees with the enhanced stability of the Pu5Rh4 type of crystal structure with increasing concentration of Sc since it has larger dGe3-Ge3 compared to the O(I) structure. The low-temperature magnetic behaviour of the alloys in the (Gd1-xScx)5Ge4 system is truly unusual. The end members of this system are not ferromagnetic: Gd5Ge4 orders antiferromagnetically at 130 K and Sc5Ge4 is a Curie-Weiss paramagnet with no magnetic ordering. However, the alloys in the middle part of the system (leaning towards the Gd-rich side of the diagram) show clear ferromagnetic behaviour (Figure 3). Near both ends of the (Gd1-xScx)5Ge4 system the alloys become antiferromagnetic. Furthermore, the ferromagnetic part of the (Gd1-xScx)5Ge4 system constitutes several distinct regions, and the compositional dependence of TC exhibits a maximum in the middle part of the diagram, highlighting the complexity of magnetic interactions (Figure 4). In addition the magnetic ordering may (first-order transitions) or may not (second-order transitions) be coupled with the structural transformations, which also evolve with x(Sc).

Figure 3. M(T) dependence for (Gd1-xScx)5Ge4 alloys with 0 ≤ x ≤ 0.3. The M(T) curves of Sc-rich alloys are not shown for clarity; their behaviour resembles that of x = 0.3 with gradually diminishing TC.

Replacing gadolinium in Gd5Ge4 with a small amount of Sc (0.025 ≤ x ≤ 0.1) leads to the emergence of the ferromagnetic state even at low magnetic fields; similar effect occurs when Ge is replaced with a small amount of Si.46 Magnetic properties of (Gd0.975Sc0.025)5Ge4 alloy are similar

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Table 1. Comparison of crystal structures and magnetic properties of Gd5Ge4-based alloys doped with 5% of either R (Sc, Y) or T (Si) element. Compound/Property

Gd5Ge4

(Gd0.95Y0.05)5Ge4

(Gd0.95Sc0.05)5Ge4

Gd5(Si0.05Ge0.95)4

Low T - 0 kOe Structure/

O(II)/AFM

O(II)/AFM

O(I)/FM+O(II)/AFM

O(I)/FM+O(II)/AFM

O(I)/FM

O(II)/AFM

O(I)/FM+O(II)/AFM

O(I)/FM+O(II)/AFM

TC , K

-

-

45

46

TN, K

128

117

105

128

Ref

32

32

This work

33

magnetic order Low T - 30 kOe Structure/ magnetic order

All alloys crystallize in O(II)-Sm5Ge4-type structure at ambient conditions. Low temperature (T) crystal structures/magnetic states are reported both for zero and 30 kOe magnetic field (H).

to binary Gd5Ge4 including a single PM-AFM transition in low magnetic fields at 130 K (not clearly visible in Figure 3) and a kinetically arrested metamagnetic magnetostructural transition at lower temperatures. However, the critical field of this O(II)-AFM – O(I)-FM transition is lower compared to Gd5Ge4 signaling that Sc doping brings a fundamentally different contribution to the magnetism of Gd5Ge4 than Lu, where the latter significantly suppresses ferromagnetism at such concentration.47 The FM state is further enhanced in (Gd0.95Sc0.05)5Ge4, where the O(II)-AFM – O(I)-FM transformation occurs at 30 K in 1 kOe applied magnetic field (Figure 3). The temperaturedependent XRD data show that as much as 50% of the O(II) phase is transformed into the O(I) phase at this temperature in zero field (the transformation completion approaches 90% with a few Tesla applied magnetic field). This behaviour is drastically different from that of (Gd0.95Y0.05)5Ge4 (see Table 1), where no FM state is observed even at 50 kOe.45 At the same time, the PM-AFM transition becomes weaker and occurs at lower TN = 105 K. The FM behaviour becomes clearly dominant in the (Gd0.9Sc0.1)5Ge4, where AFM-FM transition occurs at higher temperature (TC = 45 K), while the PM-AFM long range transition transforms into a short range order anomaly (formation of FM clusters) manifested as a strong enhancement of magnetic susceptibility around 150 K (Figure 3). Both x = 0.05 and 0.1 samples show strong thermal hysteresis near TC, approximately 9 K wide. Up to this point (x = 0.1) there are similarities between Gd5(SixGe1-x)4 and (Gd1-xScx)5Ge4 systems with low x (see Table 1). In both systems a small amount of the corresponding dopant does not yet produce significant changes in the electronic structure but removes the kinetic arrest present in Gd5Ge446 because of chemical pressure, which acts similarly to hydrostatic pressure. The key difference here is that the unusual disappearance of the AFM state with 10% Sc cannot be explained by the effect of chemical pressure alone. Most likely the difficulty of maintaining long-range order above 100 K arises from the increased

local disorder due to Sc/Gd substitution, while the shortrange order is clearly present below 150 K.

Figure 4. Magnetic phase diagram of the (Gd1-xScx)5Ge4 system. Three regions are separated by the vertical dashdoted lines: 1 – the region with both second-order AFM and first-order FM transformations; 2 – first-order PM-FM transition only, with TC raising as a function of x(Sc); and 3 – a region with conventional second-order transformation at the magnetic ordering temperature, which gradually decreasing Tc or TN with x(Sc). When Sc concentration x increases above 0.1, but remains below 0.3, the alloys show first-order FM O(I) – PM O(II) transition at TC. No AFM order is observed. The deviations from Curie-Weiss law that are typically seen in reciprocal 48,49 susceptibility above TC in R5T4 compounds and are commonly associated with both static and dynamic short-range correlations arising from the competing AFM and FM states are more pronounced in this region compared to other compositions. The most interesting physical phenomenon observed in this range is the crossover between TC and θp in the composition – critical temperature phase diagram around

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x=0.15 (Figure 4). While the Weiss temperature, which is considered a measure of the exchange interaction strength, decreases, the TC steadily increases when Gd is replaced by Sc. The enhancement of ferromagnetism due to Sc substitution is most prominent in this region. While it is unusual to see TC being considerably higher than θp, it is expected to occur in a system with a first-order transformation – the θp represents the exchange in the paramagnetic hightemperature phase [O(II), x-ray powder diffraction pattern is shown in Figure S1], while TC is a characteristic of the ferromagnetic low-temperature structure [O(I), x-ray powder diffraction pattern is shown in Figure S2]. Experimentally, the (Gd0.8Sc0.2)5Ge4 compound is on the verge of transformation from the O(II) to Pu5Rh4-type structure (see Figure 1b). As mentioned above, theoretically Pu5Rh4-type is the most stable structure for this composition and it supports ferromagnetism. The exchange interactions between the slabs within the Pu5Rh4 type (Gd0.8Sc0.2)5Ge4 are higher by 70% compared to the same in O-II and lower by 26% with respect to O-I, and are clearly positive (ferromagnetic) even if slightly lower compared to the O-I structure. The origin of ferromagnetism in these Sc-substituted alloys is related to the strong hybridization between Sc1 – 3d and Gd – 5d electrons, which is shown in Figure 5 for (Gd0.8Sc0.2)5Ge4.

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This hybridization at and below the Fermi level and spin polarization manifested as the imbalance between the integrated occupied spin up and spin down states lead to ferromagnetism with a substantial itinerant magnetic moment of 0.24 µB/Sc.

Figure 5. The Gd 5d and Sc 3d density of states (DOS) of (Gd0.8Sc0.2)5Ge4. These states are strongly hybridized at and below the Fermi level.

Figure 6. Correlation between the structural transition (illustrated by the respective x-ray powder diffraction patterns) and magnetic entropy change values calculated from M(H) data using Maxwell relations: a),b) temperature-dependent XPD patterns, c),d) magnetic entropy change in first-order (Gd0.8Sc0.2)5Ge4 and second-order (Gd0.7Sc0.3)5Ge4 alloys, respectively.

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Starting at x=0.3 and above, the θp and TC become the same within experimental error, which coincides with the change of the transition order from the first to the second. This change is coupled with the change of the structure type at room-temperature from O(II) Sm5Ge4 to Pu5Rh4 (x-ray powder diffraction pattern is shown in Figure S3) and can be predicted by analysing the dGe3-Ge3 distance.

is close to zero at -1.8 K, which appears to be a critical concentration for the shift between FM and AFM exchange. Remarkably, this shift occurs at the Sc concentration above which only one of the three available R positions is still occupied by Gd. For the binary Sc5Ge4 there is no magnetic ordering.

The magnetic ordering temperature in this part of the diagram gradually decreases with x due to a simple dilution effect. The system loses its sensitivity to external stimuli, and, consequently, some of the useful properties. For example, magnetocaloric effect (MCE) in the firstorder (Gd0.8Sc0.2)5Ge4 material with a sharp structural change at TC is much larger than the MCE in the secondorder (Gd0.7Sc0.3)5Ge4 with no first-order structural change at TC (Figure 6). As mentioned earlier the transition in (Gd0.8Sc0.2)5Ge4 is from the O(II)-Sm5Ge4 to the O(I)Gd5Si4 crystal structure and is accompanied by discontinuous changes in lattice parameters (Figures 6a and 7a) and unit-cell volume (Figure 7c) commonly observed during such transformations. The Pu5Rh4 – O(I) transition at TC in (Gd0.7Sc0.3)5Ge4 (temperature dependence of the dGe3-Ge3 distance is shown in Figure S5), on the other hand, does not involve a first-order change of unit-cell volume and the thermal evolution of lattice parameters is gradual but anisotropic, and only weakly dependent on magnetic field (Figures 6b and 7b,c). This type of a second-order crystallographic transformation between Pu5Rh4–type and O(I)-type structures has not been reported in any of the R5T4 systems in the past.9 We note that the magnetostructural transition in the (Gd0.8Sc0.2)5Ge4 sample is somewhat unusual as well. It has a distinct first-order character and the same O(II)-O(I) structural change as the alloys with lower Sc content. However, unlike those alloys, it shows little hysteresis at TC, a very desirable property for a potential magnetocaloric material. This effect (reduction of hysteresis) will be carefully investigated in the future, for it may hold a key to controlling hysteresis in other first-order materials relevant to magnetocalorics. When Sc concentration exceeds 50 %, the alloys become antiferromagnetic (for both x = 0.632 and x = 0.75, see Figures S6 and S7 in the supplementary information). Apparently, the dilution effect from Gd/Sc substitution becomes dominant and the in-slab ferromagnetism is no longer supported. It has to be noted that the AFM structure in these scandium rich alloys must be different from the AFM order in Gd5Ge4, where antiferromagnetism exists due to antiparallel alignment of ferromagnetic slabs.28 While the paramagnetic Curie temperature is largely positive in Gd5Ge4 (θp = 130 K50), it becomes negative in (Gd0.25Sc0.75)5Ge4, θp = -15 K, indicating a shift in the sign of magnetic interactions (in Figure 4, the absolute value of θp is plotted for the (Gd0.25Sc0.75)5Ge4 composition). The earlier reported θp value for (Gd0.4Sc0.6)5Ge4

Figure 7. a) Temperature dependence of lattice parameters in (Gd0.8Sc0.2)5Ge4 measured in zero magnetic field on heating and cooling; b) Temperature dependence of lattice parameters in (Gd0.7Sc0.3)5Ge4 measured on cooling in 0 and 30 kOe applied magnetic fields. Temperature dependence of the unit-cell volume for (Gd0.8Sc0.2)5Ge4 (red circles) and (Gd0.7Sc0.3)5Ge4 (blue squares) compounds.

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4. Discussion and Conclusions The study of the (Gd1-xScx)5Ge4 pseudobinary system shows that unexpected and interesting physics may arise in a complex intermetallic system even when conventional wisdom points otherwise. Both antiferromagnetic Gd5Ge4 and paramagnetic Sc5Ge4 adopt the O(II)-Sm5Ge4 type of crystal structure and based on the compositional dependence of lattice parameters and unit cell volume, the (Gd1-xScx)5Ge4 system may easily be assumed to represent a continuous solid solution. The reality, on the other hand, is very rich, easily controlled by chemistry, physics, and crystallography. The Scandium substitution leads to ferromagnetic order, initially through a first-order coupled magnetic-polymorphic transition that later transforms into a second-order transition when the Pu5Rh4 structure replaces the Sm5Ge4 structure at room temperature while the Gd5Si4 structure forms via a second order transformation at low temperatures far below TC. At higher concentrations of Sc the dilution effect from Gd/Sc begins to dominate the magnetic behaviour but the system surprisingly remains ferromagnetic until high x(Sc) = 0.75. At low Sc concentrations the appearance of the FM phase may be explained by change in kinetics, namely a removal of the “frozen” AFM state present in Gd5Ge422 with chemical pressure exerted by much smaller Sc atoms. At the same time DFT calculations predict that the ferromagnetic state of the Pu5Rh4 structure at higher Sc concentrations is stable. Broad, delocalized 3d electron states of Sc strongly overlap (hybridize) with delocalized 5d electron states of Gd, thus promoting exchange interactions between the slabs. Sc preferentially occupies the 4c site (inside the slabs) and the exchange interactions between these sites, and, ultimately, the exchange interactions between neighbouring slabs, is controlled by the dGe3-Ge3 interslab distances.29 The Ge3-Ge3 distance, even though measured at room temperature is in fact the most reliable indicator of the system’s behaviour at low temperatures. Our study proves that it may be difficult to evaluate a system’s potential for unusual phase changes based on the “normal” lattice parameters behaviour, or overlook compound’s magnetism based on the assumed non-magnetic behaviour of the substituting element; theoretical calculations may prove to be useful in this regard even if a system does not look too promising initially. Finally, this study confirms that the presence of a firstorder magnetostructural transition can greatly enhance magnetocaloric effect of a given system; the enhancement arises from the additive contributions due to nonmagnetic entropy difference between the two competing polymorphic modifications.43 Furthermore, contrary to a popular belief, such enhancement utilizing the enthalpy of structural transition is not always accompanied by large or even moderate hysteresis; thus carefully designed materials with first-order magnetic transformation must indeed be considered as the front-runners for magnetocaloric cooling and heat pumping applications. Clearly,

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scandium cannot be ignored anymore when such novel magnetic functional materials are developed.

ASSOCIATED CONTENT Supporting Information. Selected X-ray powder diffraction data (including CIFs), magnetic susceptibility data, and temperature dependence of dGe3-Ge3 interatomic distance for (Gd0.7Sc0.3)5Ge4 compound are available as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Yaroslav Mudryk, 254 Spedding Hall, Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA.

E-mail address: [email protected]. Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University of Science and Technology under contract No. DE-AC02-07CH11358. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences Division.

ABBREVIATIONS AFM, antiferromagnetic; FM, ferromagnetic; PM, paramagnetic; XPD, X-ray powder diffraction.

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The ferromagnetism in a functional alloy system, (Gd1-xScx)5Ge4 rises when a strongly magnetic element, Gd, is replaced by a nominally non-magnetic one, Sc. This unlikely emergent magnetism is coupled with unusual phase relationships and crystallography. These phenomena, neatly tuned by Sc substitutions, were both predicted and explained by first-principles theory and calculations.

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Enhancing Magnetic Functionality with Scandium: Breaking

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