Metallic Borides, La2Re3B7 and La3Re2B5, Featuring Extensive

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Metallic Borides, La2Re3B7 and La3Re2B5, Featuring Extensive Boron− Boron Bonding Daniel E. Bugaris,† Christos D. Malliakas,†,‡ Duck Young Chung,† and Mercouri G. Kanatzidis*,†,‡ †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: La2Re3B7 and La3Re2B5 have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La2Re3B7 (space group Pcca) having lattice parameters a = 7.657(2) Å, b = 6.755(1) Å, and c = 11.617(2) Å, and La3Re2B5 (space group Pmma) having lattice parameters a = 10.809(2) Å, b = 5.287(1) Å, and c = 5.747(1) Å. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron−boron bonding. La3Re2B5 features fairly common B2 dumbbells, whereas La2Re3B7 has unique onedimensional subunits composed of alternating triangular B3 and trans-B4 zigzag chain fragments. Also observed in La3Re2B5 is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La2Re3B7 is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch−Grüneisen model and a room-temperature resistivity ρ300 K of ∼375 μΩ cm. The electronic band structure calculations also suggest that La3Re2B5 is a regular metal.

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INTRODUCTION Metal borides from the ternary system RE−TM−B (RE = rareearth element; TM = transition metal) have been studied extensively for the wide variety of physical properties which they display. Perhaps the most well-known compound from this system is the hard ferromagnet Nd2Fe14B, which has found commercial application as a strong permanent magnet in electric motors and generators, computer hard disk drives, and magnetic resonance imaging (MRI) equipment. Superconductivity has been observed in a number of these phases, including RERh4B4 (RE = Y, Nd, Sm, Er, Tm, Lu),1 RETM3B2 (RE = La, Lu; TM = Rh, Os, Ir),2 (La1−xYx)Co2B2, La(Co1−xFex)2B2,3 and La3Ru8B6.4 One particularly interesting compound is ErRh4B4, where superconductivity and ferromagnetism coexist at low temperatures.5 Mixed-valence (Eu2+/Eu3+, Ce3+/Ce4+) has been reported for EuPt4B,6 CeTM3B2 (TM = Ru, Ir),7 and CeIr4B4.8 The compounds CeIr2B2,9 CePd8B2−x, and Ce3Pd25‑xB8−y10 exhibit Kondo lattice behavior, while PrIr2B2 is a spin glass.11 A large magnetocaloric effect (MCE) is found in PrCo2B2,12 which may make this a candidate material for energy-efficient and environmentally friendly magnetic refrigeration. Despite the wealth of interesting physical properties for RE− TM−B phases, some systems have only been minimally investigated. For this study, we have chosen to focus on the La−Re−B phase space. Among the binary compounds, there are two La borides (LaB4 and LaB6) and three Re borides (ReB2, Re7B3, and Re3B). LaB6 is a refractory ceramic material that possesses a low work function and a high electron emissivity, which has allowed it to find application as a hot © XXXX American Chemical Society

cathode material for electron microscopes and electron beam welders, among other instruments. ReB2 is an ultra-incompressible, superhard material13 that may be useful as an industrial abrasive/cutting tool or as a scratch-resistant coating. Both Re7B3 and Re3B are low-temperature superconductors, with transition temperatures Tc of 3.3 and 4.8 K, respectively.14 There are no known La/Re binary alloys. With regard to ternary phases in the La−Re−B system, only La2Re3B7, “∼LaRe4B4”, and “∼La3Re4B4” have been reported.15 For these phases, the full structure has been described only for La2Re3B7,16 while ∼LaRe4B4 is described only as tetragonal with lattice parameters a = 7.40 Å and c = 10.60 Å, and no structural data beyond its existence is given for ∼La3Re4B4. No physical properties or theoretical calculations have been detailed in the literature. Herein we report on the syntheses, structures, and properties of two ternary compounds, La2Re3B7 and La3Re2B5. Single crystals of these materials were grown for the first time from the elements using a La/Ni eutectic flux at 1000 °C, followed by slow cooling and centrifugation. From single-crystal X-ray diffraction, the structure of La2Re3B7 has been confirmed, albeit with higher precision, and the completely novel structure of La3Re2B5 has been determined. Both crystal structures are three-dimensional frameworks composed of Re boride polyhedra linked together through extensive B−B bonding. We provide a thorough examination of the boron subunits in Received: November 10, 2015

A

DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

EDS on several crystals of each composition using the La Lα and Re Lα lines confirmed the 2:3 and 3:2 ratios in each compound, respectively. The boron content could not be confirmed due to the limitation of EDS with elements lighter than Na. No impurity elements, such as Ni (from the flux) or Al (reagent or from the crucible), were detected in the crystals. Single-Crystal X-ray Diffraction. Single crystals of La2Re3B7 and La3Re2B5 were chosen and mounted on the tips of glass fibers for Xray diffraction. Intensity data were collected at 298 K using ω scans on a STOE 2T imaging plate diffraction system using graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and 40 mA with a 34 cm diameter imaging plate. Individual frames were collected with a 4 min exposure time and a 0.5° ω rotation. XAREA, X-RED, and X-SHAPE software packages were used for data collection, integration, and analytical absorption corrections, respectively.17 Structures were solved with the direct methods program SHELXS and refined with the full-matrix least-squares program SHELXL.18 Each final refinement included a secondary extinction correction. The parameters for data collection and details of the structure refinements are given in Table 1. Atomic coordinates and thermal displacement parameters (Ueq) are given in Table 2, and selected interatomic distances are given in Tables 3 and 4. The composition ∼La2Re3B8 was reported to crystallize with an orthorhombic crystal structure having lattice parameters a = 7.71 Å, b = 6.76 Å, and c = 11.5 Å, as determined by powder X-ray diffraction.15 A later reference described the correct composition as La2Re3B7, belonging to orthorhombic space group Pcca, with lattice parameters a

these compounds and compare them with other metal borides exhibiting various B−B structural motifs. Furthermore, electrical resistivity measurements on single crystals of La2Re3B7 display metallic behavior. Electronic band structure calculations also suggest that La2Re3B7 and La3Re2B5 are metals.

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EXPERIMENTAL METHODS

General Details. Lanthanum (Alfa Aesar, 99.9%), rhenium (Alfa Aesar, 99.99%), aluminum (Alfa Aesar, 99.97%), and boron (Alfa Aesar, 98%) were used as received. La/Ni eutectic (Alfa Aesar, 88:12 wt %) was obtained in the form of a large ingot that was broken apart into small pieces prior to use. All materials were handled in a M-Braun glovebox under an inert Ar atmosphere ( 1). Among discrete (zero-dimensional) boron-containing fragments, a wide variety of shapes/geometries have been reported, with the most common being B2 dumbbells. Some metal boride structure types that contain B2 dumbbells include CeCo4B4,48 NdCo4B4,49 LuRu4B4,50 Mo2FeB2,51 and W2CoB2.52 The next most frequent boron-containing subunit is the trans-B4 zigzag chain, which is observed in structure types such as Mo2IrB2,53 Ni3ZnB2,54 and Ti1+xRh2−x+yIr3‑yB3.55 Some other rare discrete B−B structural motifs are the cis-B4 zigzag chain (β-Cr2IrB2),56

Figure 6. Two-dimensional sheet of hexagonal rings formed by corner and edge sharing of ReB4 tetrahedra in La3Re2B5, as viewed down the b axis. F

DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the linear B4 chain (Rh5B4),57 the trigonal planar B4 unit (Ti1+xOs2−xRuB2),58 the B5-zigzag chain (Ni12AlB8),35 the cis− trans−cis-B6 chain (Rh3B2−x),59 and the planar B6 hexagonal ring (Nb6Fe1−xIr6+xB860 and Ti7Rh4Ir2B861). Infinite one-dimensional boron zigzag chains are found in both binary (FeB type,62 with Ti, Cr, Mn, and Co analogs) and ternary (Fe2AlB2 type,63 with Cr and Mn analogs) phases. Another 1D structural motif, albeit quite uncommon, is the chain of B4 distorted square links, observed only in the isostructural compounds MgOs3B4 and ScOs3B4.64 Extended B−B bonding in two dimensions can lead to infinite layers composed of 5-, 6-, and/or 7-membered rings. Examples of this include the AlB2 structure type,65 with a honeycomb lattice of planar hexagonal rings, the YCrB4 structure type,66 with pentagonal and heptagonal rings, and the Y2ReB6 structure type,67 with pentagonal, hexagonal, and heptagonal rings. One of the current compounds, La2Re3B7, contains infinite 1D boron chains, which in and of itself is not unusual. However, what is unique is that the chains are not simple zigzag chains but rather chains composed of two types of links, which are each known boron structural fragments. The B−B bond distances in the B3 triangles are 1.70(2) and 1.91(1) Å (×2), as compared to 1.814(9) Å for the B3 triangle in Be1.09B3 (the B3 unit forms a triangular “neck” in the Be6B21 cage).68 Furthermore, the B3 triangle in Be1.09B3 is equilateral (all B− B−B bond angles equal to 60°), whereas it is an acute isosceles triangle in La2Re3B7. Additionally, each boron atom in the B3 unit of Be1.09B3 is further bonded to Be and B atoms (layers) above and below it, so that it is sandwiched between them. In La2Re3B7, atom B(4) at the apex of the B3 triangle is also bonded to Re(1) while atoms B(2) at the base of the B3 triangle are each bonded to Re(1), Re(2), and B(1) of the trans-B4 zigzag chain fragment. The B−B bond distances in the trans-B4 zigzag chain fragments are 1.76(2) and 1.80(1) Å (×2), which are comparable to 1.833 (×2) and 1.855 Å in the trans-B4 zigzag chain of Ni3ZnB2. The other current compound, La3Re2B5, possesses B2 dumbbells with a B−B bond distance of 1.74(2) Å. This value is similar to those found for B−B bonds of B2 dumbbells in some other Re borides, including 1.770− 1.842 Å in Pr7(Re4B4)6 (an incommensurate derivative of NdCo4B4 type),26 1.830 Å in YRe4B4 (CeCo4B4 type), and 1.834 Å in LaRe4B4 (CeCo4B4 type).15 Electrical Resistivity of La2Re3B7. The electrical resistivity (ρ) versus temperature (T) for La2Re3B7 is shown in Figure 8. La2Re3B7 is metallic, as exhibited by the increase in resistivity

with increasing temperature. The room-temperature resistivity, ρ300 K, is ∼375 μΩ cm, which is comparable to the values observed for some other metal borides, including 121 μΩ cm for Mg3Rh5B3,69 ∼250 μΩ cm for Ru7B3,70 ∼320 μΩ cm for Mg1−xRhB,71 and ∼420 μΩ cm for Sc4Rh17B12.72 The somewhat large residual resistivity (ρ0 ≈ 100 μΩ cm) may be due to pitting in the crystal due to soaking it in dilute HCl to clean the excess flux from the surface. However, the large residual resistivity and small residual resistivity ratio (RRR = ρ300 K/ρ0 ≈ 3.75) may also indicate a high degree of intrinsic structural disorder (point defects) in the complex unit cell. The electrical resistivity data for La2Re3B7 can be fit well to the Bloch−Grü neisen model, represented by the following equation ⎛ T ⎞4 ρ(T ) = ρ0 + AT ⎜ ⎟ ⎝ θD ⎠

∫0

θD/ T

x 4 dx (e x − 1)(1 − e−x)

where T is the temperature, ρ0 is the residual resistivity, θD is the Debye temperature, x is the possible phonon frequencies of the material, and A is a scale factor. The fifth-order term in the equation represents Umklapp electron−phonon scattering, which is the main electron-scattering mechanism at temperatures higher than the impurity regime. The values of the parameters determined from the fitting to the experimental data are ρ0 = 100.7(4) μΩ cm, θD = 211(3) K, and A = 9.77(1) × 10−7 K−1. For comparison, the Debye temperatures for some other ternary metal borides, also determined from resistivity data, are 175 and 205 K for La3Pd25B8 and LaPd8B2,10 respectively. Electronic Band Structures. The DFT electronic band structure calculations of La2Re3B7 and La3Re2B5 show dispersive valence and conduction bands near the Fermi level (Figures 9 and 10). La2Re3B7 appears to have a negative band gap, where the conduction band maximum (CBM), between the X and the S points, and the valence band minimum (VBM), between the Y and the R points, do not overlap (Figure 9b). Along with the low density of states (DOS) at the Fermi level, this indicates that La2Re3B7 is a semimetal, which is consistent with our resistivity measurements (∼375 μΩ cm at 300 K for La2Re3B7). The electronic band structure for La3Re2B5 shows some overlap of the CBM and VBM (Figure 10b), along with a higher DOS at the Fermi level, suggesting stronger metallicity. The majority of the partial density of states of La2Re3B7 (Figure 9c) near the Fermi level (±1 eV) are composed of Re bands, with smaller fractions of B and La bands. The DOS of La3Re2B5 (Figure 10c) shows an almost equal contribution of La, Re, and B bands near the Fermi level (±2 eV).

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CONCLUSION The ternary rare-earth rhenium borides, La2Re3B7 and La3Re2B5, have been prepared in single-crystalline form for the first time. The crystal growth was performed at 1000 °C utilizing a low-melting La/Ni eutectic flux. The reactive La/Ni eutectic flux seems to be an excellent molten medium for overcoming the synthetic challenge of combining highly refractory elements such as rhenium and boron into multinary compositions. This has previously been shown in the crystal growth of the Os-containing borides LaOs 2 Al 2 B and La2Os2AlB2 as well.39 We will continue to apply this crystal growth protocol with the La/Ni eutectic flux to other recalcitrant elemental compositions to search for novel materials.

Figure 8. Temperature-dependent electrical resistivity of La2Re3B7 in the temperature range 5−300 K as well as the fit to the Bloch− Grüneisen model. G

DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. (a) Electronic band structure of La2Re3B7. (b) Expanded view (between −0.5 and 0.5 eV) of the electronic band structure. (c) Projected densities of states (DOS) for the various atom types in La2Re3B7.

Figure 10. (a) Electronic band structure of La3Re2B5. (b) Expanded view (between −0.5 and 0.5 eV) of the electronic band structure. (c) Projected densities of states (DOS) for the various atom types in La3Re2B5.

structural unit, an infinite boron chain with alternating triangular B3 and trans-B4 zigzag chain fragments. La3Re2B5 contains discrete boron atoms with a previously unknown octahedral coordination by rare-earth atoms (in this case La). These findings indicate that the unusual binding modes of boron may give rise to a rich structural complexity, particularly in ternary, quaternary, and higher order chemical systems.

The structures of La2Re3B7 and La3Re2B5, as determined via single-crystal X-ray diffraction, consist of unique three-dimensional frameworks. Re is bonded to six and eight B atoms in La2Re3B7, whereas La3Re2B5 contains tetrahedral ReB4 units. Although these coordination environments of B atoms surrounding transition metal atoms are common, both compounds feature some other unusual structural features. The 3D framework of La2Re3B7 is built up from a novel 1D H

DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX

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The binary rhenium boride ReB2 has been characterized as an ultra-incompressible, superhard material.13 The two-dimensional crystal structure of ReB2 features alternating layers of hexagonal close-packed (hcp) Re atoms and puckered hexagonal sheets of B atoms. From X-ray diffraction, the density of ReB2 has been determined to be 12.669 g/cm3.25b The boron−boron bonding in the current ternary compounds as well as the relatively high densities (e.g., 10.083 g/cm3 for La2Re3B7 and 8.527 g/cm3 for La3Re2B5) suggest that these materials may be good candidates for hardness testing. Temperature-dependent resistivity measurements on single crystals of La2Re3B7 reveal metallic behavior. This is supported by electronic band structure calculations showing that these compounds are poor metals or semimetals. Further exploration of the La−Re−B phase space may uncover additional compounds with interesting charge transport properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02599. Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 847-4671541. Fax: 847-491-5937. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Electron microscopy was accomplished at the Electron Microscopy Center at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC0206CH11357 by UChicago Argonne, LLC.

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DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02599 Inorg. Chem. XXXX, XXX, XXX−XXX