Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr

Article Cite This: Acc. Chem. Res. 2018, 51, 230−239

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Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr, Nd, Eu, Ca; Pn = P, As) to Their Crystal and Electronic Structures Published as part of the Accounts of Chemical Research special issue “Advancing Chemistry through Intermetallic Compounds”. Xiaoyan Tan,# Zachary P. Tener, and Michael Shatruk* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States CONSPECTUS: Rare-earth cobalt pnictides, RCo2Pn2 (Pn = P, As), belong to the ThCr2Si2 structure type, which is ubiquitous among intermetallic compounds. The structural and magnetic properties of simple ternary RCo2P2 phosphides, which combine partially delocalized (itinerant) 3d magnetic moments of cobalt and localized 4f magnetic moments of lanthanides, were investigated extensively in 1980−1990s, predominantly by the Jeitschko group. Those studies established that LaCo2P2 shows ferromagnetic (FM) ordering of Co moments, while the other members of the series, with R = Ce, Pr, Nd, or Sm, exhibit antiferromagnetic (AFM) ordering in both R and Co magnetic sublattices. This observation also correlated with the larger separation between the [Co2P2] layers in the crystal structure of LaCo2P2 as compared to the decreased interlayer distances in the other structures of the RCo2P2 series. Our work over the past decade has focused on unraveling the rich magnetic behavior that can be observed in these systems when internal chemical and external physical factors are used to perturb their crystal and electronic structures. We began our foray into these materials by demonstrating that the preservation of FM ordering of Co 3d moments in the mixed La1−xR′xCo2P2 phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La0.75Pr0.25Co2P2 such mutual influence of the 3d and 4f moments leads to a cascade of magnetic phase transitions. All these changes were traced back to the modification of the crystal structure and, consequently, the electronic band structure of these materials. The substitution of smaller R3+ ions for the La3+ ions leads to structural compression along the tetragonal c axis, perpendicular to the [Co2P2] layers, and an increase in the Co−Co distances within the layer. This structural effect is translated into more localized Co magnetic moments, stronger magnetic exchange between Co sites, and higher ordering temperatures. A more dramatic change in properties is observed in EuCo2Pn2, which exhibit AFM ordering of the localized 4f moments of Eu2+ ions and only paramagnetic behavior in the Co sublattice. Under applied pressure, these compounds undergo structural collapse, which causes a dramatic decrease in the separation between the [Co2Pn2] layers, an increase in the oxidation state of Eu, and magnetic ordering of Co moments. We further demonstrated that similar effects can be stimulated by chemical compression, which is achieved by doping Eu into the more constrained lattice sites, for example, in PrCo2P2 or CaCo2As2. In both cases, the induced mixed valence of Eu results in the change from AFM to FM ordering in the Co sublattice. A series of solid solutions Ca1−xEuxCo2As2 shows a fascinating evolution of magnetic behavior from AFM ordering of Co 3d moments to simultaneous FM ordering of Co 3d and Eu 4f moments to AFM ordering of Eu 4f moments as one proceeds from CaCo2As2 to EuCo2As2. Importantly, all these changes in magnetic properties are well justified by the analysis of electronic density of states and crystal orbital Hamilton population, providing the understanding of how chemical factors can be leveraged, in general, to modify properties of itinerant magnets.

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INTRODUCTION

atoms in a chessboard-like fashion (Figure 1). Such atomic arrangement leads to [T2X2] slabs that alternate with layers of A atoms along the tetragonal c axis. A seminal 1985 paper by Hoffmann and Zheng2 provided the first general theoretical analysis of correlations between the

ThCr2Si2 is the most common structure type among ternary intermetallic compounds, with more than 1000 representatives.1 The chemical composition of such structures corresponds to the general formula AT2X2, where A is an electropositive metal (e.g., alkaline-earth or lanthanide), T is a transition metal, and X is a metalloid or nonmetal. The crystal structure features square planar nets of T atoms capped from the top and bottom by X © 2018 American Chemical Society

Received: October 25, 2017 Published: January 5, 2018 230

DOI: 10.1021/acs.accounts.7b00533 Acc. Chem. Res. 2018, 51, 230−239

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Figure 2. Crystal structures of LaCo2P2 (a) and PrCo2P2 (b) and the alignment of Co moments.

solely by the lanthanide contraction. Thus, one might assume that the transition from LaCo2P2 to RCo2P2 with R = Ce, Pr, Nd, or Sm might be associated with instabilities in the magnetically ordered structures, such as spin reorientations and interconversions between FM and AFM states. Such instabilities should be strongly related to the electronic band structure, since all these materials are metals and the Co 3d moments are significantly delocalized, leading to the itinerant nature of magnetic ordering in the Co sublattice. Therefore, we will begin this Account with a brief discussion of itinerant magnetism, which will be followed by specific examples of our studies on RCo2P2 materials that reveal the power of applying theoretical analysis and chemical thinking to the modification of magnetic behavior in itinerant systems. We will demonstrate how a cascade of magnetic phase transition was achieved in La0.75Pr0.25Co2P2, how inducing mixed valence of Eu led to FM ordering in Pr0.8Eu0.2Co2P2, how the change in the nature of the nonmetal atom led to the modified magnetic behavior of both 3d and 4f moments in RCo2As2, how the behavior of EuCo2As2 can be strongly affected by both physical and chemical pressure, and finally how a complex interplay between the crystal and electronic structures and magnetic properties was discovered in Ca1−xEuxCo2As2. We hope that this Account will allow the reader to appreciate the complexity and beauty of magnetostructural correlations that can be achieved in these rather simple structures by a well-reasoned chemical approach.

Figure 1. ThCr2Si2 structure type, highlighting the [T2X2] layers alternating with layers of A atoms.

valence electron count in the [T2X2] layer and the structural parameters of the ThCr2Si2-type materials.2 It was shown that the increase in the electron count leads to stronger interactions between the X atoms of adjacent layers and eventual formation of interlayer X−X bonds. This analysis led Hoffman and Zheng to suggest that some AT2X2 compounds might exhibit both normal and collapsed ThCr2Si2-type structures. Indeed, both situations were later observed experimentally for SrNi2P2,3 EuCo2P2,4 and EuFe2As2,5 which showed abrupt pressure-induced structural phase transitions. In the case of EuFe2As2, the pressure-induced increase in the Eu oxidation state and concomitant electron doping into the [Fe2As2] layer led to the emergence of superconductivity below 30 K,6 similar to other FeAs-based high-temperature superconductors.7 In the quest to investigate potential correlations between the crystal structure and magnetic behavior of the ThCr2Si2-type compounds, we turned our attention to a family of rare-earth cobalt phosphides, RCo2P2 (R = La, Ce, Pr, Nd, Sm), which were investigated in detail by the Jeitschko group.8 Our interest was provoked by the distinctly different behavior of LaCo2P2 versus the other representatives of this series. LaCo2P2 exhibits a long interplanar P−P distance of 3.16 Å and ferromagnetic (FM) ordering at 132 K due to alignment of Co (3d) magnetic moments parallel to the [Co2P2] plane (Figure 2a).9 The other RCo2P2 compounds show a much shorter P−P separation, ∼2.5−2.6 Å (albeit still much longer than the single P−P bond length of 2.20 Å), and antiferromagnetic (AFM) ordering of Co moments at or above room temperature. These materials also show AFM ordering of R (4f) magnetic moments but at much lower temperatures. In contrast to LaCo2P2, in the other RCo2P2 structures, the Co moments are ordered along the c axis; they exhibit FM order in each [Co2P2] plane but alternate in opposite directions along the c axis, thus resulting in the AFM ground state (Figure 2b).10 As becomes evident from these considerations, the pronounced difference in the magnetic behavior of LaCo2P2 and the other RCo2P2 materials is accompanied by the substantial change in the crystal structure parameters, which cannot be justified

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ITINERANT MAGNETISM: A (VERY) BRIEF INTRODUCTION Due to the concise nature of this Account, we introduce only briefly the concept of itinerant magnetism while referring the reader to several reviews on this topic.11−16 Itinerant magnetism emerges in metallic systems that exhibit high density of states (DOS) at the Fermi level (EF, the highest energy of electrons in solid) and sufficient delocalization of magnetic electrons to achieve appropriate band dispersion. Both conditions are typically satisfied for systems containing d-electrons. Stoner showed that such conditions can lead to spontaneous spin polarization of the electronic band structure at the Fermi level, creating an unbalanced spin distribution, that is, magnetic ordering (Scheme 1).16 The Stoner criterion states that the spontaneous polarization occurs for the band structures with Jn(EF) > 1, where J is the nearest-neighbor magnetic exchange constant and n(EF) is the DOS at the Fermi level. Dronskowski and Landrum proposed chemical interpretation of itinerant magnetism by considering the interaction between 231


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understanding the interplay between the electronic structures and magnetism in itinerant electron systems.

Scheme 1. Spontaneous Spin Polarization of the Band Structure Leads to Itinerant Ferromagnetism

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La0.75Pr0.25Co2P2: A CASCADE OF MAGNETIC PHASE TRANSITIONS Given the contrasting properties of LaCo2P2 and PrCo2P2, as described in the Introduction (normal vs collapsed ThCr2Si2type structure, FM vs AFM ordering of Co moments), we can assume that interesting property variations might be observed in solid solutions La1−xPrxCo2P2. Indeed, the substitution of even a small amount of Pr for La results in the increase of the FM ordering temperature (TC) and appearance of another magnetic phase transition at low temperatures (Figure 4a).18 The FM-like

magnetic orbitals via crystal orbital Hamilton population analysis (COHP).11 The sign of the −COHP function reveals bonding (positive), antibonding (negative), and nonbonding (nearly zero) interactions between specified atoms.17 FM ordering is promoted by strong antibonding interactions at EF, while nonbonding interactions should favor AFM ordering. As an example, the nonpolarized DOS and COHP plots for LaCo2P2 (Figure 3) clearly demonstrate the high DOS value and Figure 4. (a) Temperature dependence of magnetic susceptibility for La1−xPrxCo2P2. (b) Polarized DOS for x = 0 and x = 0.88, with the contribution from Co 3d orbitals highlighted in red.

transition is preserved even for samples with large Pr content, for example, La0.25Pr0.75Co2P2. The increase in the ordering temperature can be justified by analyzing the changes in the crystal and electronic structures. As the Pr content x increases, the structure undergoes compression along the c axis but expands in the ab plane, which leads to an increase in the intraplanar Co− Co distances, since d(Co−Co) = a 2 . The smaller Co−Co orbital overlap leads to the decreased dispersion of the 3d band, more localized Co-based states at the Fermi level, more effective spin polarization (Figure 4b), and higher TC. As a representative of this series, La0.75Pr0.25Co2P2 offers perhaps the most interesting example of complex magnetic behavior that can emerge in the compositional space between the parent LaCo2P2 and PrCo2P2 structures. Magnetic measurements on a single crystal of La0.75Pr0.25Co2P2 revealed two pronounced magnetic phase transitions at 167 and 66 K and one weaker transition at 35 K (Figure 5a). The transition at higher temperature appears to be FM and is associated with a much larger increase in the magnetization value when the magnetic field is oriented in the ab plane as compared to the value observed with the field parallel to the c axis. Based on the reported magnetic structure of LaCo2P2,9 which undergoes FM ordering at 132 K, we conclude that the 167 K transition in La0.75Pr0.25Co2P2 is due to the FM ordering of Co moments in the ab plane (Figure 5b). At 66 K, however, the magnetization measured in the ab plane drops nearly to zero while the magnetization along the c axis slightly increases. Such behavior was not observed for any simple ternary RCo2P2 material. We used neutron diffraction and X-ray magnetic circular dichroism (XMCD) spectroscopy19 to establish that the abrupt drop in abplane magnetization is due to reorientation of the Co 3d moments toward the c axis. At the same time, Pr 4f moments

Figure 3. Nonpolarized DOS and Co−Co COHP for LaCo2P2. The red-shaded area shows the contribution from Co 3d orbitals.

antibonding Co−Co interactions at EF. Importantly, the spin polarization results in the expansion of the majority (α) and the contraction of the minority (β) spin density on the magnetic sites, as manifested by increasing interatomic distances at the point of FM ordering (the Curie temperature, TC). The spinpolarized −COHP plots for itinerant magnets reveal the decrease in the antibonding interactions at EF, indicating the overall stabilization of the electronic structure due to magnetic ordering.11 In the following sections, we will repeatedly use the Stoner and Dronskowski−Landrum criteria for justification or prediction of magnetic behavior. These criteria provide the foundation for 232


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in a different direction. Second, while the magnetic ordering temperatures for the Co and Pr sublattices in PrCo2P2 are drastically different (304 and 18 K, respectively), in La0.75Pr0.25Co2P2 the FM order in the Co sublattice appears to drive the FM ordering in the Pr sublattice, shifting it to 66 K despite the much lower Pr content as compared to PrCo2P2. Third, due to the strong local magnetic anisotropy of Pr3+ ions, FM ordered Pr moments align parallel to the c axis and drive reorientation of Co moments along the c axis. Thus, the ability to manage magnetic ordering in the Co sublattice affords the observation of rich magnetic behavior for the mixed La1−xPrxCo2P2 structures.

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Pr0.8Eu0.2Co2P2: INDUCING MIXED VALENCE TO ACHIEVE FERROMAGNETISM Having seen how substantially the magnetic behavior could be changed in the La1−xPrxCo2P2 structures, we hypothesized that the effect can be even more dramatic if one performed aliovalent (nonisoelectronic) substitution in the rare-earth sites, thus causing the change in the number of electrons formally donated to the [Co2P2] layer by the electropositive R metal. EuCo2P2 was shown earlier to contain Eu2+ ions under ambient conditions.20 Therefore, substituting Eu into a RCo2P2 structure could lead to interesting electronic effects, which might have strong ramifications on magnetic properties. A support to this hypothesis came from the properties of EuCo2P2 itself. Under ambient conditions, this material exhibits a normal ThCr2Si2-type structure, with a large interplanar P−P separation of 3.27 Å (Figure 6). Importantly, the +2 oxidation

Figure 5. (a) Temperature dependence of magnetization for a single crystal of La0.75Pr0.25Co2P2 with a magnetic field of 10 mT applied parallel and perpendicular to the c axis. (b) Magnetic structures of La0.75Pr0.25Co2P2 in different temperature ranges between the critical points of magnetic phase transitions. (c) Magneto-optical images of the single crystal of La0.75Pr0.25Co2P2 oriented with the c axis along the optical axis of the microscope.

Figure 6. Pressure-induced transition from the normal (LP) to the collapsed (HP) structure in EuCo2P2 and the similarity of the structural and magnetic properties of HP-EuCo2P2 and LP-PrCo2P2.

undergo FM ordering, but AFM coupling between the Co and Pr sublattices results in the overall ferrimagnetic (FiM) ordering. An insight into the magnetic transition observed at 35 K came from magneto-optical imaging of the domain structure of a single crystal of La0.75Pr0.25Co2P2. The transition at 66 K causes the development of labyrinth domains with the total magnetization pointing along the c axis (Figure 5c). Below 35 K, however, the intensity of the domain pattern decreased, which was attributed to the more perfect compensation of magnetization of Co and Pr sublattices, that is, mutual reorientation of the 3d and 4f magnetic moments becoming antiparallel or nearly antiparallel to each other. The cascade of magnetic phase transitions observed in La0.75Pr0.25Co2P2 allows for a few important conclusions. First, while both Co and Pr magnetic moments undergo AFM ordering in PrCo2P2, preserving the FM ordering of Co moments in La0.75Pr0.25Co2P2 causes the Pr moments to order also FM, albeit

state of Eu drastically changes the magnetic behavior of this material as compared to the properties of other RCo2P2 compounds. In EuCo2P2, the Co sublattice exhibits only Pauli paramagnetism and does not undergo magnetic ordering even at very low temperatures, while the Eu 4f moments exhibit helical incommensurate AFM ordering in the ab plane below 66 K.21 A fascinating pressure-induced transition in EuCo2P2 was discovered by Jeitschko and co-workers in 1998.4 At the critical pressure of 3.1 GPa, the structure undergoes an abrupt collapse as the interplanar P−P separation shortens to 2.57 Å, the Eu sites attain the oxidation state of +3, thus becoming nonmagnetic, and the Co sublattice develops itinerant AFM ordering at 260 K (Figure 6). It is interesting to note that the crystal structure and magnetic behavior of the high-pressure (HP) form of EuCo2P2 are very 233


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Figure 7. (a) Temperature dependence of magnetic susceptibility for Pr0.8M0.2Co2P2 (M = Eu, Ca). (b) 151Eu Mössbauer spectra of Pr0.8Eu0.2Co2P2 at 298 and 78.6 K. The blue solid line shows a Lorentzian fit. (c) L3-Eu XANES spectra of Pr0.8Eu0.2Co2P2 at various temperatures. A fit of the 8 K spectrum with a combination of Eu2+ and Eu3+ spectral components is shown.

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RCo2As2: SYNTHESIS OF NEW FERRIMAGNETIC MATERIALS At the time when we embarked on the exploration of magnetic properties of RCo2P2 materials, nothing was known about the magnetic behavior of their isostructural arsenide analogues, RCo2As2 (with the exception of EuCo2As2, which will be covered in the next section). A sole report on the synthesis and structural properties of RCo2As2 (R = La, Ce, Pr, Nd) was provided by Marchand and Jeitschko, who nevertheless could not obtain phase-pure samples of these compounds.25 We found that phasepure RCo2As2 and their representative single crystals can be prepared by a reaction between elements in Bi flux, which is removed after the reaction using centrifugation followed by washing with a 1:1 v/v mixture of H2O2 and glacial acetic acid.26 Interestingly, the crystal structure determination revealed the formation of Co vacancies, which were not encountered in the structures of RCo2P2. Furthermore, we observed partial substitution of Bi for the rare-earth metal in La0.97Bi0.03Co1.9As2 and Ce0.95Bi0.05Co1.85As2, but not in PrCo1.8As2 and NdCo1.7As2 (Table 1). The interplanar As−As separation in

similar to those of the ambient low-pressure (LP) form of PrCo2P2, which is in line with the Eu3+ oxidation state observed in HP-EuCo2P2. This incited us to ask what would happen to the Eu sites if they were introduced as dopants into the PrCo2P2 structure. Consequently, we synthesized Pr0.8Eu0.2Co2P2, which showed interplanar P−P distance of 2.58 Å under ambient pressure,22 almost unchanged from the distance observed in the LP-PrCo2P2 or HP-EuCo2P2. The magnetic behavior of Pr0.8Eu0.2Co2P2, however, turned out to be drastically different from that reported for any other RCo2P2 compound, as we observed FM ordering of Co moments at 282 K (Figure 7a). In addition, a model compound, Pr0.8Ca0.2Co2P2, also showed FM ordering at 278 K. While the ionic radii of 8-coordinate Ca2+ (1.12 Å) and Pr3+ (1.126 Å) ions are similar, the PrCo2P2 host lattice offers a better fit to the Eu3+ ion (1.066 Å) than to the Eu2+ ion (1.25 Å).23 But the observed magnetic behavior of Pr0.8Eu0.2Co2P2 suggests that the oxidation state of Eu3+ is unlikely in this compound. The resolution came from 151Eu Mössbauer spectroscopy, which revealed that Eu in Pr0.8Eu0.2Co2P2 exhibits homogeneous mixed valence, with the average oxidation state of +2.4 (Figure 7b). The single Mössbauer signal with the isomer shift of −6.4 mm/s was observed between those of EuF3 (0 mm/s) and LP-EuCo2P2 (−10 mm/s). The Mössbauer spectrum collected at 78 K showed a hyperfine splitting pattern, in agreement with the magnetic ordering in this material. The probing time scale of 151 Eu Mössbauer spectroscopy is ∼1 ns, but X-ray absorption near-edge structure (XANES) spectroscopy is a much faster technique that probes electronic states at the 1 fs time scale. As a result, the mixed-valence contributions were resolved in the XANES spectra, which showed both Eu2+ and Eu3+ components (Figure 7c). The dramatic modification of magnetic properties due to the change in the Eu oxidation state shows the crucial role of electronic effects and band structure in defining the magnetism of RCo2P2 materials. In EuCo2P2, the change in the Eu oxidation state was achieved by applying physical pressure. The mixed valence of Eu in Pr0.8Eu0.2Co2P2, however, was observed under ambient pressure and promoted by chemical compression of the Eu sites in the constrained host lattice. Using an earlier Mössbauer study of a gradual pressure-induced Eu2+ to Eu3+ transition in EuFe2P2,24 the chemical compression of the Eu sites in Pr0.8Eu0.2Co2P2 was estimated as being equivalent to ∼5.6 GPa. Such strong compression causes a transfer of ∼0.1 electron per formula unit from the Eu 4f states to the [Co2P2] layer, which is sufficient to trigger itinerant ferromagnetism.

Table 1. Properties of RCo2−xAs2 Materials Synthesized from Bi Flux

a

compound

d(As−As),a Å

TCo C , K

TRC, K

3d−4f coupling

La0.97Bi0.03Co1.9As2 Ce0.95Bi0.05Co1.85As2 PrCo1.8As2 NdCo1.7As2

2.8815(6) 2.7615(6) 2.7315(6) 2.7215(6)

178 147 140 62

∼70 ∼50 ∼30

FM AFM AFM

The interplanar As−As distance determined at room temperature.

La0.97Bi0.03Co1.9As2 is 2.882 Å, substantially shorter than the interplanar P−P distance of 3.16 Å in LaCo2P2, and the As−As separation in the other RCo2−xAsx structures is even shorter (Table 1). Thus, these structures might be considered as the collapsed ThCr2Si2 type. In all RCo2−xAs2 materials, the Co moments undergo FM ordering along the c axis. This is different from LaCo2P2, in which they order FM in the ab plane,9 and from the other RCo2P2 structures, in which the Co moments are ordered AFM along the c axis.10 The 4f moments of magnetic R3+ ions in RCo2−xAs2 undergo FM ordering at lower temperature than the TC observed for the FM ordering of Co 3d moments (Table 1). In the case of Pr- and Nd-containing compounds, an abrupt decrease in the total magnetization is observed around 50 and 30 K, respectively, in agreement with antiparallel alignment of the 3d and 4f 234


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Accounts of Chemical Research moments. Such magnetic coupling results in FiM ordering, similar to the observation made for La0.75Pr0.25Co2P2.19 Ce0.95Bi0.05Co1.85As2, however, behaves differently, showing only a small moment at the Ce sites and FM 3d−4f coupling. We conclusively established this behavior by polarized neutron diffraction, which showed the Ce magnetic moment to be only 0.20 μB per atom, that is, much smaller than the saturation magnetic moment of 2.14 μB expected for the Ce3+ 4f1 ion. The possibility of Ce4+ oxidation state was excluded by XANES spectroscopy, which showed the average oxidation state of Ce to be +3.06. The suppressed magnetic moment of Ce in Ce0.95Bi0.05Co1.85As2 suggests the strong hybridization of the 4f electron with the conduction band, which is frequently observed in Ce intermetallic compounds.27 This also explains the different sign of 3d−4f magnetic exchange in Ce0.95Bi0.05Co1.85As2 as compared to the Pr- and Nd-containing analogues (Table 1).

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EuCo2As2: PRESSURE-INDUCED ITINERANT FERROMAGNETISM In contrast to the RCo2−xAs2 materials just discussed, the properties of EuCo2As2 were reported earlier.28−30 Unlike RCo2−xAs2 structures, EuCo2As2 does not show vacancy formation in the Co sites. Its ambient (low-pressure) magnetic behavior is similar to that of LP-EuCo2P2: both materials contain Eu2+ ions and exhibit AFM ordering of the localized Eu 4f moments at 47 K28 and 66 K,20 respectively. We grew crystals of EuCo2As2 from Bi flux and determined its nuclear and magnetic structures from single-crystal neutron diffraction data.31 The Eu 4f moments lay in the ab plane and form an incommensurate helical AFM structure, which is very similar to the magnetic structure of LP-EuCo2P2.21 Similar to the pressure-induced structural phase transition in EuCo2P2 described above,4 EuCo2As2 also undergoes a structural collapse, albeit of the second order, at the critical pressure of 4.7 GPa at room temperature.29 In EuCo2P2, the structural collapse causes the transition from Eu2+ to Eu3+ and itinerant AFM ordering of Co moments, which is reminiscent of the AFM behavior of Co sublattice in the other RCo2P2 compounds with the collapsed ThCr2Si2-type structure. The RCo2−xAs2 materials, however, exhibit FM ordering of Co moments, and therefore it is logical to ask whether the same FM ordering can be developed in the Co sublattice of HP-EuCo2As2 with the collapsed ThCr2Si2-type structure, which should promote the higher oxidation state of Eu. We collected XANES and XMCD spectra at the Eu-L3 edge under variable pressure for EuCo2P2 and EuCo2As2.31 For the phosphide, a nearly complete Eu2+ → Eu3+ transition was observed (Figure 8a), which should “erase” the magnetism in the Eu sites (the ground state of the Eu3+ ions has the total angular momentum J = 0). Indeed, we observed a complete disappearance of the XMCD signal in the HP-EuCo2P2. For the arsenide, however, we observed only a partial suppression of the Eu2+ spectral component (Figure 8b), and the maximum average oxidation state of Eu reached at 12.6 GPa was +2.25, thus preserving magnetism in the Eu sublattice. Furthermore, the Eu XMCD signal was greatly enhanced under the applied pressure, suggesting the change in the nature of magnetic ordering of the Eu 4f moments. The field dependence of the Eu XMCD signal measured at 4.2 K revealed a saturation-like behavior, atypical of an antiferromagnet and characteristic of an FM or FiM ordered system (Figure 9a). Although we could not observe any XMCD signal at the K-edge of Co,32 the FM ordering of Eu 4f moments and the large increase in TC to 125 K (Figure 9b) are reminiscent of the magnetic behavior observed for RCo2As2 (R = Pr, Nd;

Figure 8. Pressure-dependent Eu-L3 XANES and XMCD spectra of EuCo2P2 (a) and EuCo2As2 (b). The XANES spectra were obtained at 10 and 300 K, respectively, while the XMCD spectra were obtained at 10 and 4.2 K, respectively. The inset in panel b shows the average Eu oxidation state as a function of pressure.

Figure 9. Dependence of the Eu-L3 XMCD signal amplitude on the applied magnetic field (a) and temperature (b) at 7.5 GPa.

Table 1). Therefore, the pressure-induced transfer of electron density from the Eu 4f orbitals to the [Co2As2] layer, likely, triggers FM ordering in the Co sublattice, which in turn induces FM ordering in the Eu sublattice. An indirect support for the hypothesis about the simultaneous FM ordering of 3d and 4f moments in HP-EuCo2As2 came from the electronic structure calculations, which revealed that the Stoner product is increased from 0.67 in the LP phase to 1.07 in the HP phase (Figure 10a), while the Co−Co interactions at EF change from nonbonding to antibonding (Figure 10b). Thus, both the Stoner and the Dronskowski−Landrum criteria for itinerant FM ordering are satisfied in the Co sublattice of HPEuCo2As2. Noteworthy, these dramatic changes in magnetic behavior are achieved with the relatively small electron transfer from the Eu 4f orbitals to the [Co2P2] layer (0.25 electron per formula unit).

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Ca1−XEuXCo2As2: UNDERSTANDING COMPLEXITY OF STRUCTURE−ELECTRONICS−MAGNETISM INTERPLAY In CaCo2As2, the magnetic moments of Co undergo AFM ordering at 76 K.33 Similar to the RCo2−xAs2 structures, this compound has a collapsed ThCr2Si2-type structure and contains Co vacancies, that is, its composition is more accurately described as CaCo1.86As2.34 Johnston and co-workers showed that the calculated energies of the AFM and FM states for this structure are very close,35 which justifies a first-order spin-flop 235


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Figure 10. DOS (a) and Co−Co COHP (b) calculated for the crystal structure of EuCo2As2 at 0 GPa (left) and 7.5 GPa (right). The contribution from Co 3d orbitals is shaded in red.

transition observed in this material at the critical applied field of 3.5 T.33,35 The itinerant AFM ordering of Co 3d moments in CaCo1.86As2 contrasts with the localized AFM ordering of Eu 4f moments in EuCo2As2, considering that the ambient-pressure structures of both materials contain electropositive metals in the same +2 oxidation state. Given the difference in the ionic radii of Ca2+ (1.12 Å) and Eu2+ (1.25 Å), one can expect chemical pressure to be exerted on Eu sites introduced into the collapsed lattice of CaCo1.86As2. Remarkably, doping just a small amount of Eu into CaCo1.86As2 triggered FM ordering of Co moments at 110 K in Ca0.9Eu0.1Co1.91As2.31 The chemical compression of the Eu site leads to a mixed-valent Eu2.18+ oxidation state estimated from XANES spectra. Following on this observation, we investigated the entire Ca1−xEuxCo2As2 series.36 With the exception of x = 0.1, as the Eu content x increases, the unit cell parameter a decreases while the unit cell parameter c and volume V increase. The intraplanar Co− Co distance changes from 2.823 Å in CaCo1.86As2 to 2.778 Å in EuCo2As2, while the interplanar As−As separation increases gradually from 2.735 Å in CaCo1.86As2 to 3.110 Å in EuCo2As2, displaying the transition from the collapsed to the normal structure. The structures of all these materials, except for EuCo2 As2, contain Co vacancies, although the vacancy concentration is higher at 0 < x ≤ 0.6 than at x ≥ 0.65. The same borderline behavior at x = 0.6 was observed for the average Eu oxidation state, which decreases from +2.17 for 0 < x ≤ 0.6 to +2.14 for x ≥ 0.65, according to analysis of L3-Eu XANES spectra collected at room temperature. This borderline behavior of the Co vacancy concentration and Eu oxidation state (Figure 11a) is also reflected in the magnetic phase diagram (Figure 11b). The initial substitution of Eu for Ca induces itinerant ferromagnetism in the Co sublattice. The FM behavior of Co 3d moments is maintained for all Ca1−xEuxCo2As2 samples with 0.1 ≤ x ≤ 0.6. Two successive magnetic phase transitions can be discerned for these materials at higher and lower temperatures, attributed to FM ordering of Co 3d moments and FM ordering of Eu 4f moments, respectively. Isothermal measurements of field-dependent magnetization at various temperatures revealed FM 3d−4f exchange, since the total saturation magnetization at 1.8 K matches the sum of the

Figure 11. (a) Average oxidation state of Eu and the concentration of Co vacancies (δ) in Ca1−xEuxCo2−δAs2 at 300 K under ambient pressure. (b) Magnetic phase diagram of Ca1−xEuxCo2−δAs2.

maximum magnetization measured at 7 T just below TCo C (at 100 K) and the theoretical contribution to the magnetization expected from the Eu sites (Table 2). This conclusion was supported by the determination of the magnetic structure of Ca0.5Eu0.5Co2As2 from single-crystal neutron diffraction data, which showed FM ordering of both Co and Eu moments along the c axis. For compounds with x ≥ 0.65, only AFM ordering of Eu 4f moments was observed (Figure 11b), while the Co sublattice remained paramagnetic at all temperatures. With the increase in the Eu content, the AFM ordering temperature increases due to the stronger AFM coupling between the Eu sites (Table 2). Single crystal neutron diffraction measurements on the sample with x = 0.7 revealed an incommensurate helical magnetic structure of the Eu 4f moments, similar to that observed for EuCo2As2. The evolution of magnetic behavior in the Ca1−xEuxCo2As2 series was examined with band structure calculations on idealized vacancy-free structures with x = 0.25, 0.50, and 0.75. For Ca0.75Eu0.25Co2As2 and Ca0.5Eu0.5Co2As2, a strong DOS peak was observed at the Fermi level (Figure 12a,b), giving the Jn(EF) products of 1.4 and 1.1, respectively, both of which satisfy the Stoner criterion of itinerant magnetism. The strong antibonding character of Co−Co interactions observed at the Fermi level (Figure 12d,e) justifies the FM ordering of Co moments for x ≤ 0.6. In contrast, the electronic structure of Ca0.25Eu0.75Co2As2 shows the Fermi level passing through a pseudogap in the DOS plot and nonbonding Co−Co states in the −COHP plot (Figure 12c,f). The calculated Jn(EF) product is only 0.3, which explains the lack of Co magnetic ordering for x ≥ 0.65.

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CONCLUSIONS Our main goal throughout this Account was to demonstrate the close relationship between the crystal and electronic structures of RCo2Pn2 pnictides and their magnetic properties. This correlation is especially vivid in Ca1−xEuxCo2As2, where the gradual modification of the electronic density of states and Co− 236


Article

Accounts of Chemical Research Table 2. Magnetic Properties of Ca1−xEuxCo2As2 nominal Eu content (x)

TCo C ,K

TEu C,K

0.1 0.3 0.4 0.5 0.6 0.65 0.7 0.9

110(2) 135(2) 150(2) 128(2) 93(2)

25(2) 38(2) 50(2) 50(2) 51(2)

TEu N,K

M100K max , μB/fu

M1.8K max , μB/fu

ΔMmax,a μB/fu

Mtheo(Eu),b μB/fu

3d−4f coupling

0.81(1) 1.09(1) 1.45(1) 1.64(1) 2.05(1)

1.40(1) 2.87(1) 3.84(1) 4.43(1) 4.99(1)

0.59(1) 1.78(1) 2.39(1) 2.79(1) 2.94(1)

0.58 1.74 2.33 2.91 3.47 3.92 4.19 5.50

FM FM FM FM FM

30(2) 32(2) 38(2)

100K b y+ 2+ ΔMmax = M1.8K max − Mmax . For the oxidation state Eu , established by XANES measurements, Mtheo(Eu) = 7x(3 − y), since Mtheo(Eu ) = 7 μB and 3+ Eu has a nonmagnetic ground state. a

Figure 12. DOS and −COHP plots for Ca1−xEuxCo2As2 with x = 0.25 (a, d), 0.5 (b, e), and 0.75 (c, f).

Present Address

Co interactions at the Fermi level results in the evolution of magnetism from AFM ordering of itinerant Co 3d moments for x = 0 to the simultaneous FM ordering of the Co 3d moments and localized Eu 4f moments for 0 < x ≤ 0.6 and finally to the AFM ordering of Eu 4f moments for 0.65 ≤ x ≤ 1. In the R1−xR′xCo2P2 and RCo2As2 materials with magnetic rare-earth metal ions, the promotion of itinerant ferromagnetism in the Co sublattice induced FM ordering in the rare-earth sublattice, which has never been observed in the simple RCo2P2 structures with AFM ordered Co and R moments. Fascinating changes in magnetic properties of Eu-containing pnictides have been observed under action of both physical and chemical pressure, showing the unifying effect of these factors on the electronic structure and magnetic behavior. Importantly, many of these changes could be justified or even foreseen by the analysis of the electronic band structure of materials. We believe that further exploration of these magnetostructural correlations can lead to new exciting discoveries in this and other families of intermetallic materials.

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Xiaoyan Tan: Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854. Notes

The authors declare no competing financial interest. Biographies Xiaoyan Tan obtained her M.S. degree from University of Science and Technology of China. She carried out her graduate research on itinerant magnets and magnetocaloric materials in the Shatruk laboratories at Florida State University (FSU) and defended her Ph.D. thesis in 2016. She is currently a postdoctoral researcher with Prof. Martha Greenblatt at Rutgers University. Zachary P. Tener graduated from Iowa State University in 2015 with a B.S. degree in chemistry. He is currently pursuing his Ph.D. studies in the Shatruk group at FSU, where his research focuses on investigation of correlations between itinerant magnetism and crystal and electronic structures of solids.

AUTHOR INFORMATION

Michael Shatruk is a Professor of inorganic and materials chemistry at FSU. Originally from Lviv, Ukraine, he obtained his Ph.D. degree from Lomonosov Moscow State University in 2000, under supervision of Prof. Andrei Shevelkov. After postdoctoral stints with Prof. Stephen Lee at Cornell University and Prof. Kim Dunbar at Texas A&M University, he began his independent research program at FSU in 2007. His main

Corresponding Author

*E-mail: [email protected]. ORCID

Michael Shatruk: 0000-0002-2883-4694 237


Article


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research interests include itinerant magnetism and magnetocaloric effect in intermetallic compounds and spin-state switching in molecular materials. His work was recognized with the NSF CAREER award and ACS ExxonMobil Solid State Faculty Fellowship.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation for the support of this research program through the CAREER (DMR-0955353) and regular (DMR-1507233) research grants, as well as the Florida State University for the startup funds that helped to initiate this program. We also thank the Oak Ridge National Laboratory (ORNL) sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy, and the Advanced Photon Source at Argonne National Laboratory (ANL), for providing us with the beam time and scientific support for advanced measurements that were performed at these unique research facilities. Finally, we express most sincere gratitude to Dr. Kirill Kovnir (currently at Iowa State University) and Dr. Corey Thompson (currently at Purdue University), who helped to begin this research program in the Shatruk group, and to Dr. V. Ovidiu Garlea and Dr. Huibo Cao (ORNL), Dr. Alexey Menushenkov and Dr. Alexander Yaroslavtsev (Moscow Engineering Physical Institute), and Dr. Daniel Haskel (ANL), who provided the indispensable expertise in neutron scattering and X-ray absorption spectroscopy techniques. The achievements described in this Account would have been impossible without participation of these outstanding scientists.

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Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr

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