Investigation of Hardness of Ternary Borides of the YCrB4, Y2ReB6

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Investigation of Hardness of Ternary Borides of the YCrB4, Y2ReB6, Y3ReB7, and YMo3B7 Structural Types Georgiy Akopov,† Hang Yin,† Inwhan Roh,† Lisa E. Pangilinan,† and Richard B. Kaner*,†,‡,§

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Department of Chemistry and Biochemistry, University of California, Los Angeles (UCLA), Los Angeles, California 90095, United States ‡ Department of Materials Science and Engineering, University of California, Los Angeles (UCLA), Los Angeles, California 90095, United States § California NanoSystems Institute (CNSI), University of California, Los Angeles (UCLA), Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: The metal borides: YReB4, YCrB4 (YCrB4 structural type), Y 2 ReB 6 , Sc 2 ReB 6 (Y 2 ReB 6 ), Y 3 ReB 7 , Y3MoB7, and Y3WB7 (Y3ReB7), and YMo3B7 (YMo3B7) structural types, and their solid-solutions: YRe1−xCrxB4, Y1−xScxCrB4, Y2−2xSc2xReB6, Y2Re1−xCrxB6, and YMo1−xWxB7 were analyzed using powder X-ray diffraction and energydispersive X-ray spectroscopy, and studied for their mechanical properties. These metal borides possess unique crystal structures, not found in any other type of borides: alternating metal and boron layers with boron being arranged in 5-, 6-, and 7-member rings (YCrB4, Y2ReB6), corrugated cages of 5and 13-member boron rings (Y3ReB7), as well as stacked layers of ribbons of hexagonal boron atoms 6 rings wide (YMo3B7). Although none of these borides in pure form possess high hardness, their solid solutions are superhard (Vickers hardness above 40 GPa): 42.48 ± 2.13 GPa for an alloy with a nominal composition of (YRe0.5Cr0.5):4B, 42.02 ± 2.05 GPa for (Y0.5Sc0.5Cr):4B and 41.33 ± 2.18 GPa for (YScRe):6B, at 0.49 N of applied load. As research on binary systems has become increasingly saturated, these results suggest that there are great opportunities to explore the potentially exciting properties of ternary and higher borides.

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INTRODUCTION Metal borides are a unique class of compounds that possesses a large variety of crystal structures1 that are rivaled only by their interesting and exciting mechanical,1,2 optical, magnetic, and electronic properties.3−7 Binary metal borides (one metal and one boron) and their solid solutions (e.g., HfB2 (P6/mmm) and TiB2 (P6/mmm) forming Hf1−xTixB2 (P6/mmm)1 have been extensively studied for their mechanical (and other) properties.8−16 Therefore, there is a need to investigate ternary (two metals and boron) and higher metal boride structures that are completely different from their binary boride parents, which might possess superhardness or other interesting properties. Looking at the numbers, binary metal borides crystallize in ∼26 unique crystal structures among ∼224 different metal borides and encompass ∼1000 entries in the ICSD (Inorganic Crystal Structure Database),17 while ternary metal borides have ∼2800 entries. Although it is difficult to summarize the extent of different crystal structures for ternary metal borides, we can safely assume that it is 2−3 times more than for binaries, therefore, there are many structures worth investigating. Furthermore, the placement of two or more metals in distinct crystallographic sites provides a broader complexity of boron © 2018 American Chemical Society

bonding motifs than that found in binaries. As the current design rules for superhard metals involve the search for incompressible metals alloyed with boron, the varied boron coordination and polyhedra found in ternaries afford a more boron-centric approach to new structural materials. One can imagine that any boron planes must contort to accommodate the two vastly different crystallographic sites with two metal atoms of differing size. In this manuscript, we focus on four different ternary metal boride structures: YCrB 4 , 18−27 Y 2 ReB 6 , 18,19,22,24,28−30 Y3ReB718,20,23,31−33 and YMo3B7.19,34−36 Among these references, the vast majority are either phase diagram reports or crystal structure analyses, with the exception of two papers, which explore the physical properties of some of the ternary borides.30,36 YCrB4 and Y2ReB6 (both orthorhombic, Pbam) resemble a modified diboride structure, having a total metal to boron ratio of 1:2. Their crystal structures are similar to layered AlB2 (P6/ mmm), however, instead of the boron atoms being arranged in Received: July 16, 2018 Revised: August 30, 2018 Published: August 31, 2018 6494

DOI: 10.1021/acs.chemmater.8b03008 Chem. Mater. 2018, 30, 6494−6502

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Chemistry of Materials

Figure 1. Crystal structures of: (left) AlB2 (P6/mmm, ICSD (Inorganic Crystal Structure Database) 193381),54 (middle) YReB4 (Pbam, ICSD 615278),27 and (right) Y2ReB6 (Pbam, ICSD 16187).28 For AlB2, aluminum atoms are shown in dark-blue, for the other two structures: yttrium atoms are shown in blue-gray, rhenium in violet; for all structures boron atoms are shown in green. Note that each can be considered a layered structure. In AlB2 the boron atoms are arranged in sheets of hexagons, however, in the other two structures the boron atoms are arranged in 5-, 6- and 7-membered rings.

Figure 2. Crystal structures of: (left) Y3ReB7 (Cmcm, ICSD 64595)32 and (right) YMo3B7 (Pnma, ICSD 243846).36 Yttrium atoms are shown in bluegray, rhenium and molybdenum in violet; for all structures boron atoms are shown in green. For the first structure the metal atoms are arranged in trigonal prisms (top left) and the boron atoms are arranged in corrugated cages of 5- and 13-member rings (metal prisms omitted for clarity), while for the second structure, the boron atoms are arranged in stacked hexagonal bands, six hexagons wide and infinite in length, while the metal atoms are arranged in chains.

metal and one small metal to form. The first metal position, where the metal nestles in the 6- and 7-membered rings, can be occupied by most group 3 metals (Sc, Y, and La, ra = 1.65, 1.80, and 1.95 Å, respectively)37 as well as most lanthanides (ra = 1.75−1.85 Å),37 while the second metal position, where the metal nestles in the 5-membered ring, can be occupied by most group 6−8 metals: Cr, Mo, W, Mn, Re, Fe, Ru, and Os (ra = 1.30−1.45 Å), 37 with a few exceptions in the metal

planar sheets of hexagons, the boron layers are formed by 5- and 7-member rings for YCrB4 and 5-, 6- and 7-member rings for Y2ReB6 (Figure 1). However, these structures cannot be considered as solid solutions of YB2 and MB2, since the parent diborides both have a hexagonal unit cell with 6-member boron rings, while the ternary boride has an orthorhombic unit cell. The general trend appears to be that the metals that can form these phases are primarily dictated by size, requiring one large 6495

DOI: 10.1021/acs.chemmater.8b03008 Chem. Mater. 2018, 30, 6494−6502

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Figure 3. Powder XRD patterns of alloys with a composition of (YRe1−xCrx):4B, where x = 0.05, 0.25, 0.50, 0.75, and 0.95. A single phase solid solution of YRe1−xCrxB4 (Pbam) can be observed at all concentrations of chromium. The peaks were assigned using YCrB4 (Pbam, JCPDS 03−065−6902) and YReB4 (ICSD 615278).27 The figure shows a 2θ range from 0−50° (the full PXRD patterns are provided in the Supporting Information section, SI Figure S1).

combinations, according to the available literature: YCrB418−27 and Y2ReB6.18,19,22,24,28−30 Y3ReB7 and YMo3B7 (both orthorhombic, Cmcm and Pnma, respectively) are examples of the importance of the metal composition on the structure of the resulting metal boride. The first structure contains boron atoms that form corrugated cages, while the second structure has boron atoms forming stacked hexagonal bands (6-ring-wide and infinite in the third direction; Figure 2). Similar to YCrB4 and Y2ReB6, Y3ReB7 can have group 3 and lanthanide metals in the first metal position and various group 6−7 metals in the second metal position. However, for YMo3B7, while the first position can be occupied by group 3 metals and lanthanides, the metal in the second position has to be molybdenum, according to previous reports.34−36 In this work, we have found several new boride phases that can form in these structures, that have not been previously discussed in the literature as either standalone phases or as part of solidsolution stabilization. As these structures have only been identified as part of ternary phase diagrams or as crystal structures, in this paper we explore the hardness of these ternary phases as well as their solid solutions.

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getters were then placed inside the arc-melter and the chamber was evacuated under vacuum (t = ∼15−20 min) and then flushed with argon gas, repeating the process for a minimum of 4 times. The samples were melted until molten using a current I > 70 A (usually ∼140 A), flipped and then rearced in order to ensure homogeneity. Prepared samples were split in half using a tool steel Plattner-style diamond crusher, with one-half being crushed into a sub 325 mesh ( 40 GPa), compared to 34.25 ± 1.67 and 37.5 ± 2.29 GPa for pure (YRe):4B and (YCr):4B, respectively. As this composition forms a single-phase solid solution of YRe1−xCrxB4 (Figures 3 and 8), the hardness increase is reached at 50/50 at. % chromium substitution in rhenium, and therefore can be attributed to solid-solution strengthening. Here having two metals of different atomic size occupy the same atomic positions adds localized strain and perturbs the lattice uniformly (as this system is a solid solution, the lattice would have an intermediate size compared to the pure phases, and as such would be too big for one metal and too small for the other (Table 1)), which impedes dislocation propagation, thus reducing the effect of plastic deformation.50,51 Similarly, for alloys with a nominal composition (Y1−xScxCr):4B (Figure 7b), the hardness increases to 42.02 ± 2.05 GPa at low load (0.49 N), making it superhard, at 50/50 at. % scandium substitution, compared to 37.5 ± 2.29 GPa for pure (YCr):4B. This hardness enhancement can also be attributed to solid-solution hardening, as the only phase present is a solid solution − Y1−xScxCrB4 (Figures 4 and 8). This hardness increase at 50% loading is expected for solid solution hardening, as a 1:1 ratio of two varied metal atoms should result in the most localized strain. This also suggests that there is no extrinsic hardening/composite effects. The lack of secondary phases is corroborated with electron micrographs, which generally show a single phase material. For alloys with a nominal composition of (Y2−2xSc2xRe):6B (Figure 7c), the hardness 6500

DOI: 10.1021/acs.chemmater.8b03008 Chem. Mater. 2018, 30, 6494−6502

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superhard, compared to their hard parent phases. This hardness increase can be attributed to the effects of solid-solution hardening. For the YMo 3 B 7 system, a solid solution YMo2.8W0.2B7 exists and is stable under ambient pressure. Although not superhard, this composition has a hardness of ∼6 GPa more than its parent phase, 34.37 ± 3.08 vs 28.78 ± 2.31 to GPa at low load, again attributed to solid solution hardening. Thus, investigating ternary and higher borides opens up a great realm of opportunities, as the numerous phases with unique structures may possess interesting mechanical and other properties.16

increase at 50/50 at. % substitution of scandium in yttrium is 41.33 ± 2.18 GPa at low load, making it superhard, compared to 35.69 ± 2.08 and 34.74 ± 1.84 GPa for (Y2Re):6B and (Sc2Re):6B, respectively. For this system the hardness increase can be attributed to not only solid-solution hardening, but also to precipitation hardening (due to the formation of YReB4 alongside Y2−2xSc2xReB6 (Figures 5 and 8)), where formation of a secondary phase beyond the limit of solubility further impedes the movement of dislocations.52 For alloys with a nominal composition of (Y2Re1−xCrx):6B (Figure 7d), although the hardness initially increases for 5 at. % Cr, this is followed by a dramatic decrease with higher concentrations of chromium. This can be explained by the fact that the Y2CrB6 phase does not exist and the resulting solidsolution of Y2Re1−xCrxB6 only exists until ∼50 at. % Cr; therefore, the system becomes less stable. For alloys with a nominal composition of (Y3M):6B (M = Re, W, and Mo) (Table 3), the hardness differs greatly when going from Re to Mo to W: 34.31 ± 2.39, 29.29 ± 1.85, and 16.79 ± 0.47 GPa at low load, respectively. These compositions form Y3MB7 compounds, alongside secondary phases (SI Figure S5). This implies that atomic size and valence electron count play a greater role in the mechanical properties of Y3MB7 phases, whereas Re and W have virtually the same atomic radius (1.35 Å),37 they differ by 1 valence electron (6 for W vs 7 for Re); at the same time Mo, although differing in electron count by the same 1 electron, is larger than Re at 1.45 Å.37 (M)Mo3B7 (M = Y and lanthanides) phases are not known to form for any other metal in the secondary position except for molybdenum.34,35 Moreover, as most of the lanthanides have the same atomic radii, their substitution for yttrium would have limited effect. However, it was found that it is possible to substitute molybdenum for tungsten up to 0.2 equiv, forming the YMo2.8W0.2B7 alloy (SI Figure S5, Figure 8, and Table 3). Furthermore, for this composition, the hardness increases from 28.78 ± 2.31 to 34.37 ± 3.08 GPa at low load, which can be attributed to solid-solution hardening, through the disruption of the stacking of the boron ribbons in the structure. Figure 8 shows the SEM images for alloys with a nominal composition: (YRe1−xCrx):4B, (Y2−2xSc2xRe):6B, (Y3M):6B (M = Re, Mo, and W) and (YMo3−xWx):8B. In the case of (YRe1−xCrx):4B and (YMo3−xWx):8B, only a single phase can be seen, while for (Y2−2xSc2xRe):6B and (Y3M):6B (M = Re, Mo, and W) secondary phases appear alongside the main phases: YReB4 in the first case and Y2ReB6, YMoB4, and YWB4, respectively, in the second case. SI Figure S7 plots the Vickers hardness of the parent compounds and the hardest solidsolutions versus applied load, showing that the hardness of these compounds approaches the asymptotic level at higher applied loads.

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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.chemmater.8b03008. Full PXRD patterns for (YRe):4B, (YCr):4B, (YRe1−xCrx):4B (x = 0.05, 0.25, 0.50, 0.75, and 0.95), (Y1−xScxCr):4B (x = 0.05, 0.25, 0.50, 0.75, and 0.95), (Y2Re):6B, (Sc2Re):6B, (Y2Cr):6B, (Y2−2xSc2xRe):6B (x = 2.5, 12.5, 25.0, 37.5, 50.0, 62.5, 75.0, 87.5, and 97.5), (Y2Re1−xCrx):6B, (Y3Re):6B, (Y3W):6B, (YMo3):8B, and (YMo3‑zWz):8B (z = 0.2, 0.4, 0.6, 0.8, and 1.0). Hardness vs applied load graphs for (YRe 1 − x Cr x ):4B, (Y1−xSc2xCr):4B, (Y2−2xScxRe):6B, (Y2Re1−xCrx):6B (x = 0.00, 0.50, and 1.00) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Georgiy Akopov: 0000-0001-9399-9850 Richard B. Kaner: 0000-0003-0345-4924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Michael T. Yeung for helpful discussions; and the National Science Foundation Division of Materials Research, Grant DMR-1506860 (R.B.K.) and UCLA Graduate Division Dissertation Year Fellowship (G.A.) for financial support.

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REFERENCES

(1) Akopov, G.; Yeung, M. T.; Kaner, R. B. Rediscovering the Crystal Chemistry of Borides. Adv. Mater. 2017, 29, 1604506. (2) Fokwa, B. P. T. Borides: Solid-State Chemistry. Encycl. Inorg. Bioinorg. Chem. 2014, 1−14.1 (3) Samsonov, G. V.; Markovskii, L. Y.; Zhigach, A. F.; Valyashko, M. G. Boron, Its Compounds and Alloys [in Russian]; House of the Academy of the Sciences Ukrainian SSR: Kiev, 1960. (4) Samsonov, G. V.; Vinitskii, I. M. Refractory Compounds [in Russian]; Atomizdat: Moscow, 1975. (5) Samsonov, G. V.; Serebriakova, T. I.; Neronov, V. A. Borides [in Russian]; Atomizdat: Moscow, 1975. (6) Buschow, K. H. J. Magnetic Properties of Borides. In Boron and Refractory Borides; Matkovich, V. I., Ed.; Springer Berlin Heidelberg: Berlin, 1977; pp 494−515. (7) Scheifers, J. P.; Zhang, Y.; Fokwa, B. P. T. Boron: Enabling Exciting Metal-Rich Structures and Magnetic Properties. Acc. Chem. Res. 2017, 50 (9), 2317−2325. (8) Akopov, G.; Yeung, M. T.; Turner, C. L.; Mohammadi, R.; Kaner, R. B. Extrinsic Hardening of Superhard Tungsten Tetraboride Alloys

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CONCLUSIONS In this manuscript we have explored the mechanical properties of ternary metal borides of the YMB4, Y2ReB6, Y3ReB7, and YMo3B7 structure type. Each of the structures possesses an unique arrangement of boron atoms not found in any other boride types: 5-, 6-, and 7-membered rings of boron in a layer, and/or 5-and 13-boron corrugated cages and stacked ribbons of hexagonal boron atoms. For YMoB4 and YMo3B7, single-phase solid solutions were synthesized: YRe1−xCrB4, Y1−xScxB4 and YMo3−xWxB7. For the first two solid solutions, the hardness increased to 42.48 ± 2.13 and 42.02 ± 2.05 GPa at low load, for 50/50 at. % of Cr or Sc, respectively, making these compositions 6501

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DOI: 10.1021/acs.chemmater.8b03008 Chem. Mater. 2018, 30, 6494−6502