Is orthorhombic iron tetraboride superhard?

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Is orthorhombic iron tetraboride superhard? Qianqian Wang a, Julong He a, Wentao Hu a, Zhisheng Zhao b, Chao Zhang a, Kun Luo a, Yifei Lu¨ a, Chunxue Hao a, Weiming Lu¨ a, Zhongyuan Liu a, Dongli Yu a, Yongjun Tian a, Bo Xu a,* a

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, China b Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA Received 23 December 2014; revised 19 January 2015; accepted 30 January 2015 Available online 1 April 2015

Abstract Millimeter-sized FeB4 bulks were synthesized under high pressure of 15 GPa and 1600 C. Multiple analysis methodologies including XRD, SEM, TEM, and Raman spectroscopy verified this new phase with an oP10 crystal structure. A relatively low hardness of 15.4 GPa, which is consistent with hardness estimated from both macroscopic and microscopic hardness models, excludes FeB4 as a superhard material. Magnetization and resistance measurements do not indicate superconductivity in FeB4 for temperature as low as 2.5 K. Previously reported superconductivity in FeB4 needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations. © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Transition metal boride; High pressure high temperature; Superhard materials; Superconductivity

1. Introduction The introduction of covalent-bond-forming boron atoms into dense transition metals can induce profound influences on their chemical, electronic, and mechanical properties. These unique characteristics endow transition metal borides (TMBs) considerable technological applications, for example as refractory materials, thermionic emitters, and steel strengthening agents. Especially, TMBs are subjects of intensive research in the quest for novel superhard and ultra-incompressible materials [1e8]. Such efforts are related to the high valence electron density of TMBs underlying low compressibility, and the formation of strong covalent bonds generating low plasticity and high hardness [3]. Up to now, several TMBs have been synthesized and claimed to be superhard, such as ReB2 [1], OsB2 [7], and WB4 [2,8], although none of them shows a comparable (asymptotic) hardness larger than 40 GPa [3,4]. In addition to possible superhard materials, superconductivity has also been a focus of metal boride research [9e13]. * Corresponding author. E-mail address: [email protected] (B. Xu). Peer review under responsibility of The Chinese Ceramic Society.

Kolmogorov et al. predicted an orthorhombic iron tetraboride (oP10-FeB4, space group Pnnm) with a potential superconductivity (transition temperature of 15e20 K) via a combination of ab initio high-throughput and evolutionary searches [14]. Bialon et al. later suggested that this FeB4 phase might be synthesized under high pressure [15]. Most recently, this oP10-FeB4 was successfully synthesized under high pressure and high temperature (HPHT) conditions [16]. Besides the predicted BCS superconductivity, a very high hardness of 62 GPa was also reported [16]. The combined features of superhardness and superconductivity, however, are unusual to both superhard materials and superconductivity communities [17], which need further confirmation. Here we report our investigations of HPHT synthesis of FeB4 as well as its mechanical and physical properties. We synthesized millimeter-sized FeB4 and verified this FeB4 phase with an oP10 structure. Subsequent microhardness measurement on our oP10-FeB4 sample, however, excluded it as a superhard material. Additionally, magnetization and resistance measurements did not show superconductivity for temperatures down to 2.5 K.

http://dx.doi.org/10.1016/j.jmat.2015.03.004 2352-8478/© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Q. Wang et al. / Journal of Materiomics 1 (2015) 45e51

2. Material and methods 2.1. HPHT experiments FeB (98% powder, Alfa Aesar) and amorphous B (9496% powder, Alfa Aesar) were pressed into wafers and arranged alternatively with a stoichiometric ratio of 1:3 for HPHT experiments. Two high pressure apparatus were employed, namely a six-anvil apparatus for 2 GPa and 6 GPa syntheses, and a two-stage large-volume multi-anvil system for 15 GPa synthesis. The synthetic conditions are 1200e1900 C and 215 GPa with various durations of 15e180 min. 2.2. Characterizations The structure, composition, and morphology of the synthetic samples were characterized by multiple analysis methodologies including XRD (Rigaku D/max 2500/PC, Cu Ka, l ¼ 1.5406 Å), SEM (Hitachi S-3400) equipped with EDS, TEM (JEOL JEM-2010) equipped with SAED and EELS, and Raman spectroscopy (Renishaw inVia, l ¼ 514 nm). Vickers hardness measurements were performed with a microhardness tester (KB 5 BVZ). Magnetization and resistance measurements were performed with a physical property measurement system (PPMS-9T, Quantum Design). 3. Results and discussion Products from HPHT experiments performed at lower pressures (i.e. 2 GPa) or temperatures out of the range of 1200e1900 C were dominated by FeB and/or FeB49 with no indication of new phase formation. At higher pressures and intermediate temperatures (for example, 1652 C, the melting point of FeB intending for a solideliquid reaction), X-ray diffraction (XRD) patterns of the final products showed extra peaks which could not be attributed to any known phase in the FeeB phase diagram, indicating the formation of a new iron boride phase. Typical XRD patterns from the products of FeB/ B mixtures after HTHP experiments reaction are presented in Fig. 1. We note FeB phase persists even with prolonged reaction duration and increased B content in the starting material. Nonetheless, new diffraction peaks are observed and their intensities (relative to those from FeB) increase with elevated synthetic pressure. The new peaks match well with the Bragg positions of the theoretical proposed oP10-FeB4 (or oI10FeB4) phase [14]. The deduced lattice parameters for this new phase (a ¼ 4.546 Å, b ¼ 5.348 Å, and c ¼ 2.999 Å) are consistent with the theoretical ones (a ¼ 4.521 Å, b ¼ 5.284 Å, and c ¼ 3.006 Å) [14]. A Rietveld refinement of this new phase, however, is impracticable due to the mixture phases of the final product as well as the low intensity of the diffraction peaks. Other characterization techniques must be resorted to for a structure determination. Fig. 2 shows scanning electron microscopy (SEM) micrographs from typical products prepared under 6 GPa and 15 GPa. The backscattered electron imaging mode was employed to provide image with contrast as a function of

Fig. 1. XRD patterns of samples synthesized under different HPHT conditions. The tags in the bottom indicate the calculated Bragg positions of FeB (blue ones) and oP10-FeB4 (black ones). The peaks from FeB4 are marked with inverted triangles. The peak marked with square is from FeB49 phase.

chemical composition. The sample prepared under 6 GPa is dominant with three intermixed phases (Fig. 2a), which are characterized by different gray scales in the SEM images. Energy dispersive X-ray spectroscopy (EDS) analyses of these regions indicate all of them are composed of Fe and B elements, with varying Fe:B ratios of ca. 1:1 (light gray regions), 1:4 (gray regions), and 1:49 (dark gray regions). We thus identify these phases as FeB, FeB4, and FeB49, which are consistent with XRD observations. It should be mentioned that unreacted boron may also exist in the final product, which is hard to distinguish from FeB49 phase. Fig. 2b shows the detailed SEM image of the selected area in Fig. 2a, in which FeB4 crystals with well-defined crystal shapes can easily be recognized. The typical size of FeB4 phase in the 6 GPa sample is less than 40 mm. Under higher synthetic pressure, i.e. 15 GPa, all the aforementioned phases are identified in the final product. However, their distribution and fraction are quite different from those in the 6 GPa sample. Higher synthetic pressure can greatly promote the formation of FeB4 [15]. As shown in Fig. 2c, FeB4 is now the primary phase in the product, with moderate FeB and much reduced FeB49. Fig. 2d shows the detailed SEM image of the selected area in Fig. 2c, in which FeB4 is obviously the dominated phase. A millimeter-sized FeB4 bulk (the lower half of Fig. 2c) is achieved under 15 GPa, which is preferred for sequential hardness, magnetization, and resistance measurements. Transmission electron microscopy (TEM) measurement results for this new FeB4 phase are shown in Fig. 3. Fig. 3a shows a bright field image of a FeB4 crystal. The corresponding electron energy loss spectroscopy (EELS) result clearly indicates the presence of B and Fe in the crystal with no obvious contamination from other elements (inset to Fig. 3a). A high resolution TEM (HRTEM) image of FeB4 taken alone the [001] zone axis is shown in Fig. 3b with the


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Fig. 2. SEM images taken with the backscattered electron mode of two samples containing FeB4 phase. (a) A sample synthesized at 6 GPa and 1750 C showing intermixed phases of FeB (light gray), FeB4 (gray), and FeB49 (dark gray). (b) An SEM image with higher magnification for the area selected in panel a. (c) A sample synthesized at 15 GPa and 1600 C showing major FeB4 phase. (d) An SEM image with higher magnification for the area selected in panel c.

Fig. 3. TEM measurements of FeB4 phase. (a) A bight field TEM image of FeB4 crystal. The inset shows EELS result. (b) An HRTEM image of FeB4 taken along the [001] zone axis. The corresponding SAED pattern is shown as the inset. (c) and (d) SAED patterns along the [101] and [110] directions, respectively. The arrows in panel c and d mark the second-order diffraction spots.

corresponding selection area electron diffraction (SAED) pattern as the inset. SAED patterns projected along the [101] and [110] zone axes are shown in Fig. 3c and d, respectively. An orthorhombic structure is revealed with lattice parameters of a ¼ 4.602 Å, b ¼ 5.365 Å, and c ¼ 3.025 Å, which are consistent with XRD and theoretical results. Similar to the

structure determination for CrB4 [18], we simulated the SAED patterns of FeB4 in both oP10 and oI10 structures along [101] and [110] zone axes (Fig. 4). The simulated SAED patterns of oP10-FeB4, with extra diffraction points that are absent in oI10-FeB4, match the experimental observations nicely. Further evidence of the oP10-FeB4 phase is provided from the

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Fig. 4. Experimental selected area electron diffraction (SAED) patterns along the [101] and [110] zone axes compared with simulated SAED patterns along [101] and [110] directions of oP10- and oI10-structured FeB4. The consistence between experimental data and oP10-FeB4 simulated SAED patterns is obvious. The blue arrows denote second-order diffraction spots.

Raman spectroscopy measurement (Fig. 5). Therefore, the new phase is determined as oP10-FeB4 with a Pnnm symmetry, consistent with the theoretical prediction [14]. Under 15 GPa and 1600 C, we successfully synthesized millimeter-sized FeB4 (Fig. 2c) feasible for microhardness, magnetization, and resistivity measurements. We first performed the load-dependent hardness measurements, and the results are shown in Fig. 6: The measured hardness decreases with increasing load (indentation size effect), and approaches an asymptotic value of ca. 15.4 GPa. It is generally accepted that the real hardness of a material should be assigned to the

Fig. 5. Experimental Raman spectrum of FeB4 measured under ambient conditions. Calculated positions of Raman-active modes of oP10-FeB4 and oI10-FeB4 are shown as black and blue tags, respectively.

asymptotic value achieved at high loads without obvious cracks [19]. The measured asymptotic hardness value of 15.4 GPa is much lower than the nanoindentation hardness of 62 GPa reported by Gou et al. [16]. To clarify this controversy, we estimated the hardness of FeB4 with our microscopic hardness model considering contributions from metallicity and d-orbital electrons [20], which is descripted in Appendix. The calculated hardness (18.4 GPa) is consistent with the experimentally measured hardness (15.4 GPa) of our sample, and challenges the superhardness of FeB4 claimed previously [16]. This argument is further supported by recent theoretic investigations of FeB4 [21e23], all

Fig. 6. The Vickers hardness, HV, of FeB4 as a function of applied load. The HV of FeB4 decreases from 23.3 GPa at 0.49 N to 15.8 GPa at 2.94 N. Error bars indicate the standard deviations (n ¼ 5). The inset shows an optical micrograph of the Vickers indentation at a load of 0.98 N.


of which doubted the superhardness of FeB4. It should be mentioned that similar controversy also existed for isostructural oP10-CrB4. A theoretical hardness of 48 GPa was originally predicted [18]. However, experimentally determined hardness is only 23 GPa [24], which is explained later theoretically [25]. Considering the iso-structure and similar lattice parameters possessed by CrB4 and FeB4, it is unlikely that hardness of FeB4 (62 GPa) overwhelms that of CrB4 (23 GPa) [16,24]. Based on these evidences, we conclude that oP10FeB4 is not a superhard material. Fig. 7 shows the temperature dependent magnetization and resistance of our FeB4 sample. The zero-field cooled FeB4 demonstrates no diamagnetic signal and therefore no bulk superconductivity for temperature as low as 2.5 K (Fig. 7a). Also, the resistance shows a metallic behavior from room temperature to 30 K. Below that, the resistance is almost constant without superconducting transition (Fig. 7b). Our observations of no signal of superconductivity in oP10-FeB4 for temperatures down to 2.5 K is inconsistent with previous theoretic prediction [14] and experiment results [16]. Multiple possibilities may lead to this divergence. First, we used

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9496% high-energy amorphous boron and 98% FeB as starting materials to prompt the growth of FeB4 phase. The contained impurities (e.g. Cr, Mn, Mg, etc.) may substitute into the FeB4 lattice and hamper the emergence of superconductivity (or defer it to a lower temperature than 2.5 K), although no apparent impurity was revealed in EELS measurements (See the inset to Fig. 3a). Second, our sample showed a better crystallinity compared with that from previous synthetic sample [16]: Planar defects are absent in our HRTEM observations along [001] zone axis (Fig. 3b) whereas present in Fig. 1b of Ref. 16. Previous works on MgB2 and NbB2 showed superconductivity is sensitive to the crystal lattice strain and defects [26e31], which might contribute to the observed superconductivity in oP10-FeB4 [16]. Moreover, we noted another case of trace of superconductivity observed in FeeB composition spread films very recently [32]. However, this superconducting phase is compositional undetermined (oS8-FeB referred from HRTEM in Ref. 32). The origin of superconductivity in FeeB compounds thus needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations. 4. Conclusion Millimeter-sized FeB4 bulks were synthesized under high pressure of 15 GPa. Multiple structural analysis methodologies identify this new phase with an oP10 crystal structure. A relatively low hardness value of 15.4 GPa excludes FeB4 as a superhard materials. Resistance and magnetization measurements do not indicate FeB4 superconductivity for temperature as low as 2.5 K. The recently reported superhardness and superconductivity in FeB4 need a further investigation. Acknowledgments This work was supported by the National Science Foundation of China (51421091 and 51332005), and the Natural Science Foundation for Distinguished Young Scholars of Hebei Province of China (E2014203150). Q.Q.W. acknowledges the support from Science Foundation of Yanshan University for the Excellent Ph.D. Students (YSUSF201201).

Appendix.

Fig. 7. (a) The magnetic moment vs. temperature plot for oP10-FeB4. (b) The resistance vs. temperature plot for oP10-FeB4. The inset shows magnified plot under 50 K.

To evaluate the hardness of FeB4 with our microscopic hardness model [20], the bonding configuration in oP10FeB4 was first checked by the DFT calculation. Only bonds with positive populations are considered for hardness estimation. As a result, half B atoms are 5-coordinated (B5) with the other half 6-coordinated (B6), all Fe atoms are 4coordinated (Fe4). B and Fe atoms contribute 3 and 8 valence electrons, respectively. For a multi-bond system of FeB4, the hardness can be calculated as the average of hardness of all individual chemical bonds, i.e. BeB and FeeB bonds, by using

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Y 1 m m nm HV ¼ ðHV Þ

P

nm

m

;

where HVm is the hardness of a binary compound composed of m-type bond, nm is the multiplicity of m-type bond in the unit cell [20]. Table A.1 shows the hardness calculation details for FeB4. Table A.1 Calculation bond parameters and Vickers hardness of oP10-FeB4 (Z ¼ 2). Bond type

dm (Å)

nm

Nem

fim

fmm (103)

HVm (GPa)

Hv (GPa)

B5eB6 B5eB6 B5eB6 B6eB6 Fe4eB5 Fe4eB6

1.696 1.834 1.878 1.995 1.991 2.088

4 8 4 2 4 4

0.571 0.451 0.420 0.318 0.833 0.694

0 0 0 0 0.779 0.843

1.55 1.55 1.55 0.79 2.48 2.21

25.69 18.05 16.22 15.39 20.04 15.72

18.42

Hardness for FeeB bonds (with d-electron) is estimated with 2=3 1:191f m 32:2ðf m Þ0:55 . m 2:5 m i HVm ðGPaÞ ¼ 1051 Nem e ðd Þ ; where " ðdm Þ3 =

P nv ðdv Þ3 v

U

#

Nem ¼ nme =ym ¼ ½ZAm =NC;A þ ZBm =NC;B = is the valence electron density of m-type

bond. nme and ym are the number of valence electrons and bond volume of m-type bond, respectively; ZAm ðZBm Þ and NC,A (NC,B) are the valence electron number and coordination number of atom A (B), respectively; U is the volume of unit cell [20]. fmm is the metallicity factor of m-type bond [20]. fim is the bond ionicity of m-type bond estimated with Levine's scheme [33]. Hardness for BeB bonds (without d-electron and fim ¼ 0) is estimated with 2=3 32:2ðf m Þ0:55 . m 2:5 m HVm ðGPaÞ ¼ 350 Nem e ðd Þ ½20:

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Qianqian Wang received her PhD from Yanshan University (2015). Her research interests include design metastable materials and synthesis novel materials under high pressure and high temperature.

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Bo Xu is a professor of College of Materials Science and Engineering, Yanshan University. He received his PhD from University of NebraskaeLincoln (2002). After postdoctoral work at University of Maryland, College Park, he joined State Key Laboratory of Metastable Materials Science and Technology, Yanshan University in 2006. His research interests include superhard materials and thermoelectric materials.