Hydrogen-Insertion-Induced Itinerant Ferromagnetism in

Jun 11, 2019 - It is known that an intermetallic, Zr2Co adopting an Al2Cu-type crystal structure, exhibits superconductivity at Tc = 5.6 K and can ena...
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Cite This: J. Phys. Chem. C 2019, 123, 14964−14968

Hydrogen-Insertion-Induced Itinerant Ferromagnetism in Zr2CoH4.8 with Co Chains Hiroshi Mizoguchi,*,† Satoru Matsuishi,† Sang-Won Park,‡ and Hideo Hosono*,†,‡ Materials Research Center for Element Strategy and ‡Laboratory for Materials Research, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

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ABSTRACT: It is known that an intermetallic, Zr2Co adopting an Al2Cu-type crystal structure, exhibits superconductivity at Tc = 5.6 K and can enable topotactic hydrogen insertion. Although hydrogen insertion into alloys/ intermetallics of transition metal often prevents ordering of magnetic spin, Matar has theoretically predicted ferromagnetic spin ordering in Zr2CoH5 based on Stoner conditions [Matar, S. F., Intermetallics 2013, 36, 25]. We confirmed intercalation/deintercalation of hydrogen into Zr2Co at ∼300 °C, keeping the parent Al2Cu-type structure. We have succeeded in experimentally identifying a ferromagnetic transition in Zr2CoH4.8 at Tc = 128 K, as well as superconductivity in Zr2Co. The magnetization at 2 K was ∼0.3μB, although it did not saturate up to 5.5 kOe, even at 2 K. On the basis of analysis of magnetization curves, we confirmed the weak itinerant ferromagnetism of Zr2CoH4.8 derived from spin fluctuation. This hydride has an isolated Co chain running along the tetragonal c axis with a dCo−Co intrachain distance of 2.824(7) Å, which is longer than that in metallic hcp-Co (∼2.5 Å). Band structure calculations of paramagnetic Zr2CoH5 suggested that a moderate Co dz2−Co dz2 interaction in the chain plays an important role, similar to that in Pt-chain electronic conductors.

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INTRODUCTION Hydrogen insertion into alloys/intermetallics has been studied extensively for applications such as hydrogen storage materials. Furthermore, there is interest in modifying the physical and chemical properties of these materials,1−3 for example, by altering their magnetic properties by insertion of hydrogen. Whereas hydrogen insertion often prevents ordering of magnetic spin in transition-metal hydrides,4 enhancement of magnetic ordering has rarely been reported in the compounds of nf-electron systems, including Th6Mn23H30, UNiGeH1.2, and NdCoSiH.5−7 The Al2Cu structure (tetragonal, space group I4/mcm, No. 140) often appears in intermetallics or borides, and more than 40 binary A2B phases have been recognized to date.8 In this family, Al2Cu is well known to be a θ phase among metallurgists. The nucleation of the embryo of the θ phase near room temperature plays an important role in the age hardening of Duralumin, Al-4 wt % Cu alloy. Here, we consider Zr2Co belonging to the family. Similar to the charge transfer seen in Zintl phases, Zr2Co includes an anionic Co ion, owing to the difference in electronegativities9 of the two metal atoms, while the size ratio of two metal atoms is expected to be a significant factor for the formation of the Al2Cu-type crystal structure. Zr2Co is also known to absorb hydrogen easily.10−13 Figure 1a shows the crystal structure of Zr2Co reported in the literature.14 The Zr2 sublattice is composed of two honeycomb planes (Figure 1b). These honeycomb planes, located on mirror planes perpendicular to the [110] direction, cross each other at a right angle to form an interpenetrating network. The tunneling site in the Zr2 network is occupied by a Co chain running along the c axis. The Co−Co distance in the chain is © 2019 American Chemical Society

Figure 1. (a) Crystal structure of Zr2Co. The structure is composed of a Zr2 interpenetrating network and Co chains. (b) Honeycomb lattice of Zr, which forms the Zr2 sublattice. (c) Crystal structure of Zr2CoD5.

∼2.734(2) Å, which is much longer than that (∼2.5 Å) in hcpCo. In this crystal structure, the Zr2 and Co layers are alternately stacked along the c axis, as shown in Figure 1a. However, the electronic structure of Zr2Co does not have a two-dimensional (2D), but rather a 3D structure, as expected from the short Zr−Zr distances within the honeycomb lattice along the c direction, which indicates strong chemical bonding. Received: April 23, 2019 Revised: May 25, 2019 Published: June 11, 2019 14964

DOI: 10.1021/acs.jpcc.9b03793 J. Phys. Chem. C 2019, 123, 14964−14968

Article

The Journal of Physical Chemistry C In 2013, Matar15 theoretically predicted ferromagnetic spin ordering in Zr2CoH5 based on Stoner conditions.16 He investigated possible spin alignments and predicted ferromagnetism with a saturation magnetization of 0.7μB per Co. However, there have been no experimental reports concerning the physical properties of Zr2CoHx to date. Herein, we report the ferromagnetism of Zr2CoH4.8 having a Co chain at Tc (Curie temperature) = 128 K, which is induced by topotactic hydrogen insertion.

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EXPERIMENTAL SECTION

The starting materials were Zr (Rare Metallic, Japan, 99.9%) and Co (Rare Metallic, Japan, 99.97%). The Zr2Co was prepared from a stoichiometric mixture of Zr and Co via arc melting on a water-cooled Cu hearth under a high-purity Ar atmosphere. These materials were heated at 423 K under ∼0.5 MPa hydrogen for 5 h to insert hydrogen. The chemical compositions of the products were determined with a JXA8530F electron microprobe analyzer (JEOL). The hydrogen contents were estimated by thermal desorption spectroscopy (TDS). The crystal structures of the synthesized materials were determined by powder X-ray diffraction (Bruker D8 Advance), with the use of Cu Kα radiation. Rietveld refinements of the powder XRD patterns were performed using the code TOPAS4.17 The dependence of the electrical resistivity on temperature was measured over the range of 1.8−305 K by a conventional four-probe method, with the use of Ag paste to form the electrical wiring. Magnetization measurements were performed with a SQUID vibrating sample magnetometer (Quantum Design). Magnetization curves were collected by lowering the field (H) from 55 kOe to zero. Band structure calculations were performed by the linear muffin-tin orbital (LMTO) method, with the atomic sphere approximation, including the combined correction.18 k-space integrations employed the tetrahedron method using 196 irreducible k points within the Brillouin zone.

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Figure 2. (a) Powder XRD patterns of Zr2CoHx. The inset shows an SEM image of Zr2CoH4.8 powder. (b) TDS at a specific mass-tocharge (m/z) of 2 (H2) for Zr2CoH4.8. (c) Relationship between unit cell volumes, c/a ratio, and H content in Zr2CoHx.

positive character, suggesting the negative valence state of D ions. Thus, we assume that the insertion of H ions into interstitial sites causes positive hole-doping. The Co−Co intrachain distance changes slightly from 2.734(2) to 2.824(7) Å by the D insertion. In contrast, the interchain distance of the hydride (a/21/2 = 4.8979(1) Å) is long. According to our XRD measurements, Zr2CoH4.8 decomposed into ZrCo and ZrHx by heat treatment above 623 K. Hydrogen deinsertion of Zr2CoH4.8 was attempted by annealing at 523−573 K with a Ti getter in an evacuated SiO2 ampoule. The XRD measurements of the obtained Zr2CoH2.6 and Zr2CoH3.6, as shown in Figure 2a, revealed the systematic shift of lattice parameters. Table 1 summarizes the lattice parameters and unit cell

RESULTS AND DISCUSSION

Powder XRD diffraction measurements confirmed that the arcmelted Zr2Co sample has a tetragonal Al2Cu-type crystal structure, as shown in Figure 2a. The hydride was also synthesized easily, which is consistent with a previous report.12 Hydrogen embrittlement enhanced the formation of a fine powder, with particle sizes in the range of 1−30 μm, as shown in an SEM photo (inset of Figure 2a). The much lower melting point19 of Zr2Co (1394 K) than those of Co (1768 K) and Zr (2128 K) indicates the relative instability of the intermetallic phase, suggesting that the hydride of Zr2Co is stable, according to Miedema et al.’s rule.20 The hydrogen content of the hydride was determined to be Zr2CoH4.8 by TDS measurements, as shown in Figure 2b. The powder XRD pattern of the hydride was also indexed to the tetragonal cell. The crystal structure of Zr2CoD∼5 (tetragonal, space group P4/ncc, No. 130) based on powder neutron diffraction has been reported12,13 and is shown in Figure 1c. According to the literature,12 the crystal symmetry decreases on D insertion from I4/mcm (tetragonal, Z = 4) to P4/ncc (tetragonal, Z = 4). Two mirror planes convert to c- and n-glide planes. The D ions insert topotactically into the Co layer to form a pseudo-planar CoD24 polyhedron. D1 (in the 4b site) or D2 (in the 16g site) locates in the center of the Zr4 or Zr3Co tetrahedron, respectively. D ions coordinate mainly with Zr ions with a

Table 1. Tetragonal Unit Cell Parameters and Unit Cell Volumes of Zr2CoHx x

a (Å)

c (Å)

V (Å3)

V/V(x = 0)

c/a

0 2.6 3.6 4.8

6.3629(4) 6.6521(2) 6.8128(5) 6.9159(4)

5.5155(4) 5.5013(1) 5.5322(5) 5.6314(4)

223.31(3) 243.44(1) 256.77(4) 269.35(3)

1 1.09 1.15 1.21

0.867 0.827 0.812 0.814

volumes of obtained Zr2CoHx. As shown in Figure 2c, the volumes changed monotonically with x, suggesting topotactic hydrogen intercalation/deintercalation under a mild condition (T < 573 K) in Zr2CoHx . The c/a ratio decreased anisotropically by hydrogen incorporation. Figure 3a shows the resistivity−temperature (ρ−T) curve for Zr2CoHx. The sample with x = 0 exhibited a metallic temperature dependence, and the resistivity at 300 K was 2 × 10−3 Ω cm. The ρ decreased as T decreased, and a sharp drop in ρ was observed at T = 5.6 K, which indicated a superconducting transition, agreeing with a literature report.21 The sample of x = 4.8 also exhibited a metallic behavior, whereas the value at 300 K was an order of magnitude higher. We observed a change of the slope at ∼150 K. Figure 3b shows 14965

DOI: 10.1021/acs.jpcc.9b03793 J. Phys. Chem. C 2019, 123, 14964−14968

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The Journal of Physical Chemistry C

determine the Curie temperature (Tc), we used Arrott plots (M2 vs H/M plots) in the vicinity of Tc, as shown in Figure 4b. Straight lines passing the origin were obtained at T = 128−130 K, and Tc was determined to be 128 K. Spontaneous magnetization (Ms) and magnetic susceptibility (χ0) were evaluated from the extrapolation into H = 0 in the Arrott plots and are shown in the inset of Figure 5. By the extrapolation of

Figure 3. (a) Temperature (T) dependence of the electrical resistivity (ρ) of Zr2CoHx. Powders of the hydride were pressed into highdensity pellets for the measurements. (b) Temperature dependence of the magnetic susceptibility and its inverse.

the temperature dependences of magnetic susceptibility (M/ H) and (M/H)−1 of the sample with x = 4.8. As T decreased, the M/H values increased sharply at ∼130 K, exhibiting a transition from the paramagnetic to ferromagnetic state. Notably, topotactic hydrogen insertion induced itinerant ferromagnetism in Zr2Co with Co chains, while long-range spin ordering in insulating solids containing a chain of magnetic ions is suppressed by thermal fluctuation, according to the Mermin−Wagner theorem of the Heisenberg chain.22 The ferromagnetism increased gradually as T decreased but did not saturate even at 1.8 K. The decrease of the ρ of Zr2CoH4.8 in the region T < 150 K shown in Figure 3a is consistent with the magnetic ordering. Figure 4a shows the magnetization−field curves (M−H curves) at various temperatures, which indicate soft ferromag-

Figure 5. Rhodes−Wohlfarth plot including Zr2CoH4.8 (red circle). The inset shows the temperature dependence of spontaneous magnetization (Ms) and magnetic susceptibility (χ0) of Zr2CoH4.8, estimated from the extrapolation of the M2−H/M curves.

the Ms−T curve to T = 0 K, the spontaneous magnetization at the ground state Ps was determined to be 0.315μB, which agrees reasonably with the value (0.7μB) calculated by Matar.15 In terms of the origin of the magnetic spin, Zr rarely shows spin ordering, as observed in ZrZn2 with Tc = 26 K.24 However, the observed Tc (128 K) appears to be too high for Zr 4d electrons, indicating spin ordering of Co 3d electrons. Alloys/intermetallics of Co often show ferromagnetism. For examples, hcp-Co and SmCo5 have saturation magnetism values of 1.7 and 2.2μB per Co,25 respectively, which are much greater than that of Zr2CoH4.8 (∼0.3μB). As shown in the inset of Figure 5, χ0−1 showed a linear relationship with temperature in the region 138 K < T < 170 K, which is consistent with the spin fluctuation theory (i.e., self-consistent renormalization theory (SCR theory)).23 The data in the temperature region were fitted by the Curie−Weiss equation χ0 = χ1 + C/(T − θ) to give C = 0.204 emu K Oe−1 mol-Co−1 and θ = 131 K. The positive θ value indicates ferromagnetic interactions between Co spins. The effective μB (Peff) calculated from the C value was 1.31μB. We compared this value with the theoretical effective μB, Peff calculated from the equation Peff = 2[S(S + 1)]1/2, to obtain 1.73 for S = 1/2 (d9). Therefore, the experimental Peff value for x = 4.8 is slightly smaller than 1.73. The observed small values of Peff and Ps also agree well with the characteristics of weak itinerant ferromagnetism.26,27 The parameter Pc, the number of magnetic spins deduced from the C value, was calculated from the equation Pc(Pc + 2) = Peff2, and the obtained parameters are summarized in Table 2. Figure 5 shows a Rhodes− Wohlfarth plot28,29 for magnetic compounds, which are classified into two groups. For compounds having localized magnetic spins such as Gd, the Rhodes−Wohlfarth ratio Pc/Ps

Figure 4. (a) Magnetization−field (M−H) curves and (b) Arrott plots of Zr2CoH4.8 at various temperatures.

netic behavior at the region T < 120 K, with small hysteresis, having a mostly saturated magnetization (∼0.3μB). The magnetization increased gradually up to H = 55 kOe, without saturating, even at 2 K. This behavior suggests weak itinerant ferromagnetism (WIF) derived from spin fluctuation,23 and Zr2CoH4.8 is one of the few hydrides that exhibit weak itinerant ferromagnetism. The saturation magnetization changed from 0.3 to 0.4 BM, depending on the hydrization conditions of the samples. To clarify the mechanism of observed ferromagnetism, we investigated the detail of magnetization behaviors. To

Table 2. Magnetic Parameters of Zr2CoH4.8 Obtained from Magnetization Measurements

14966

Tc (K)

θ (K)

Ps (μB)

Peff (μB)

Pc (μB)

Pc/Ps

Peff/Ps

128

131

0.315

1.31

0.65

2.1

4.2

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cally in the E−k diagram. The small energy splitting (ca. 2.5 eV) is reasonable considering the relatively long Co−Co intrachain distance (2.824(7) Å). This result is reminiscent of direct Pt−Pt interactions in Pt-chain electrical conductors.33 Co−Co interactions via H ions are unlikely because the H ion does not locate between Co ions, according to the crystal structure of the hydride12 shown in Figure 1c. The weak charge transfer from Zr to Co expands the atomic orbitals of Co. We clarified that Zr2CoHx is a unique system, which exhibits superconductivity for x = 0 and ferromagnetism for x = 4.8. Direct Co−Co interactions influence the WIF properties. Hence, we consider the correlation between the Co−Co covalent interactions and magnetic 1D spin. We will further examine the origin of the magnetic structure in this hydride in our future work.

is close to 1. For WIF compounds such as ZrZn2, the ratio is greater than 1. For Zr2CoH4.8, the ratio of 2.1 is located under the empirical curve of the weak itinerant ferromagnetism group. According to the 2D-SCR theory, lowering of the dimensions of the alignment of magnetic spins decreases the ratio.30,31 Although the 1D-SCR theory has not been established as far as we know, the observed ratio might originate from the Co chain in Zr2CoH4.8. To gain insight into the influence of hydrogen on chemical bonding, we investigated the electronic band structures. Figure 6 shows the electronic band structure (E−k diagram) and

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CONCLUSIONS It is known that the intermetallic, Zr2Co adopting an Al2Cutype crystal structure, exhibits superconductivity at Tc = 5.6 K. The results of this study about the hydrogenation of Zr2Co are summarized as follows: (1) We first indicated topotactic deintercalation of hydrogen into Zr2CoH4.8 at relatively low temperatures (∼300 °C), as well as intercalation. (2) We have succeeded in experimentally identifying a ferromagnetic transition in Zr2CoH4.8 at Tc = 128 K, although hydrogen insertion into alloys/intermetallics including transition metal often prevents ordering of magnetic spin. (3) The magnetization at 2 K was ∼0.3μB, although it did not saturate up to 5.5 kOe, even at 2 K. On the basis of analysis of magnetization curves, we confirmed the itinerant ferromagnetism of Zr2CoH4.8 derived from spin fluctuation. (4) This hydride possesses an isolated Co chain running along the tetragonal c axis with a dCo−Co intrachain distance of 2.824(7) Å. Band structure calculations for nonmagnetic Zr2CoH5 suggested that a moderate Co dz2−Co dz2 interaction in the chain plays a crucial role in this metallic hydride, similar to that in Pt-chain electronic conductors.

Figure 6. Band structure diagram for Zr2CoH5: Γ = (0, 0, 0), X = (1/ 2, 0, 0), M = (1/2, 1/2, 0), Z = (0, 0, 1/2), R = (1/2, 0, 1/2), and A = (1/2, 1/2, 1/2). Calculations were performed for four molecules in the unit cell. The energy scale is defined so that the Fermi energy (EF) corresponds to zero energy. The fatband diagram on the left-hand side shows the orbital contributions of Co dz2 (blue). Schematic orbital interactions at the Γ point related to Co dz2 are also added. The Brillouin zone is shown in the inset of the E−k diagram. The PDOS is shown on the right-hand side.

density of states (DOS) for nonmagnetic Zr2CoH5. The obtained DOS agreed with the reports of Matar.15 The space group P4/ncc (No. 130) is nonsymmorphic containing translational symmetries, resulting in the band sticking at the edge of the Brillouin zone (Z-R-A-M-X line). According to the electronegativities (Zr: 1.22, Co: 1.70, H: 2.20),9 the pDOSs of Zr or Co are located at shallower and deeper energy regions, respectively, and 20 bands of H 1s appear in the region from −10.4 to −4.2 eV. These results also indicate the presence of a H− valence state. The H− ion acts as a strong σ donor and pushes up the metal s bands preferentially through H 1s−metal ns covalent interactions because the large radial distribution function of metal ns orbitals and orbital symmetry enhance s−s interactions. The covalent interactions of H 1s with the metal ns are much greater than those with the metal (n − 1)d. These are characteristic orbital interactions seen in the electronic structure of hydrides.32 As a result, the remaining d bands of Co dominate the valence band region (0 to −3.7 eV). Two flat bands are located at the Fermi energy (EF), which form a large DOS (EF), and two other bands with a large dispersion also cross EF. On the basis of the Stoner model, Matar pointed out that electronic instability caused by the large DOS (EF) is the origin of the magnetic ordering.15 This effect can be explained through covalent interaction within the Co chain. For the Co dz2 bands (blue fatbands in the E−k diagram), direct Co−Co interactions are possible at the Γ point; dz2−dz2 σ bonding and σ* antibonding states are seen at −2.5 and 0.0 eV, respectively. The orbital interactions along the chain are shown schemati-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +81-45-924-5128 (H.M.). *E-mail: [email protected]. Tel: +81-45-924-5009 (H.H.). ORCID

Hiroshi Mizoguchi: 0000-0002-0992-7449 Satoru Matsuishi: 0000-0001-8905-0255 Sang-Won Park: 0000-0002-2843-9803 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Takashi Honda (KEK) for his fruitful discussion. This study was supported by the MEXT Element Strategy Initiative to form Core Research Center. H.H. acknowledges a Grant-in-Aid for Scientific Research (17H06153) from MEXT, Japan. H.M. was also supported by the JSPS through a Grant-in-Aid for Scientific Research (18 K05270). 14967

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DOI: 10.1021/acs.jpcc.9b03793 J. Phys. Chem. C 2019, 123, 14964−14968