Synthesis and Mechanical Character of Hexagonal Phase δ−WN

Article pubs.acs.org/IC

Synthesis and Mechanical Character of Hexagonal Phase δ−WN Changchun Wang, Qiang Tao, Shushan Dong, Xin Wang, and Pinwen Zhu* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China ABSTRACT: In this work, high-quality bulk WC-structured WN (δ−WN) was synthesized via an untraditional method and the structure was accurately determined by X-ray diffraction and Rietveld refinement. In the process of synthesizing δ−WN, W2N3 and melamine were used as tungsten source and nitrogen source, respectively. The result of successfully synthesized high-quality δ−WN indicates that our method is an effective route for synthesizing high-quality bulk δ−WN and melamine is a pure nitrogen source for introducing the nitrogen to the metal precursor. The mechanical properties, bulk modulus, and Vickers hardness (HV) were first investigated by in situ high-pressure X-ray diffraction and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of δ-WN is 373 ± 8.3 GPa, which is comparable to that of c-BN. The Vickers hardness is 13.8 GPa under an applied load of 4.9 N. It is worth noting that W−W metallic bond and W−N ionic bond are mainly chemical bond in δ−WN based on the analysis of electron localization function (ELF), density of states (DOS), and Mulliken population. This result can well clarify that δ−WN is only a hard material for the lack of strong W−N covalent bonds to form 3D network structure. Our results are helpful to understand the hardness mechanism and design superhard materials in transition-metal nitrides.

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more information on δ−WN. In addition, Kawamura also synthesized δ−WN as a main product with less impurity.11 However, it is difficult to measure the hardness and mechanical properties of as-synthesized δ−WN because of its small grain size. The lack of reports about δ−WN is ascribed to the fact that high-quality TMNs are hard to be synthesized. In the process of synthesizing TMNs, the greatest challenge is to restrain the as-synthesized product decomposition. In conventional methods (gaseous nitrides, vapor deposition, and epitaxial growth),12−14 the obtained products are always present in the form of thin films and also possess a poor crystalline form due to the decomposition problems often encountered in these methods. Thus, to propose an effective way to control the decomposition of as-synthesized sample is a key factor to synthesize bulk and high-quality TMNs. Recently, a high-pressure and high-temperature (HPHT) method was used to synthesis metal nitrides with boron nitride (BN) and ternary metal oxide AxMyOz (A = alkaline or alkalineearth metal and M = main group or transition metal).15 In the process of synthesis, the high pressure was proven to be an effective way to suppress the as-synthesized samples undergoing degassing. But pure δ−WN could not be obtained using this method only through changing the conditions of temperature and pressure. δ−WN is a low-temperature phase which can be synthesized at low temperature using regular high-energy activity of nitrogen source, such as NH3 or N2H4.7 However, in HPHT method, only the mixed phases of small amount of δ−WN and W3N4 wer obtained even at a high

INTRODUCTION Recently, transition-metal nitrides (TMNs) have drawn much attention for their excellent physicochemical properties, such as low compressibility, high hardness, high temperature stability, wear resistance, and corrosion resistance.1−3 Thus, TMNs have been widely used as hard wear protective coatings, diffusion barriers in microelectronics, and optical or decorative coatings.4,5 A great effort is currently focused on the synthesis and characterization of transition-metal nitrides. In the binary tungsten−nitrogen (W−N) system, several tungsten nitrides have been reported such as rhombohedral r-W2N3, hexagonal h-W2N3, W2N, W3N4, cubic WN, and hexagonal WN.6−8 It is believed that the hardness of materials with extended covalent bonding is higher than that of materials with linearly distributed bonding.9 Hexagonal-phase tungsten mononitride (δ−WN) is of particular interest because it adopts an unusual P6̅m2 structure (No. 187) with 3D cation−anion network, indicating “disordered” atomic arrangements with highly directional bonds.10 δ−WN is thus structurally more resistant against shear deformation than other phase tungsten nitrides and, hence, has a potential to achieve higher hardness. However, δ−WN has been rarely studied and there is a paucity of reports about the mechanical properties of this nitride. About 50 years ago, Schöenberg claimed that he had synthesized δ−WN crystal (space group No. 187, lattice parameters a = b = 2.893 Å and c = 2.826 Å and α = β = 90° and γ = 120°).7 But no further studies on δ−WN have been reported since then. Very recently, Wang et al. claimed that they successfully synthesized δ−WN at high pressure and high temperature.8 Because they only obtained the mixed phases of c-W3N4 and a small amount of δ−WN, there was not much © XXXX American Chemical Society

Received: December 15, 2016

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DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Characterizations. The phase analysis of the as-synthesized sample was examined by X-ray diffraction (XRD) using a Rigaku D/ max-2500 X-ray diffraction with Cu Ka (l = 1.5404 Å) radiation. The collected data from X-ray diffraction were analyzed by the Rietveld method using the GSAS program.16,17 The refinement was done with Shifted Chebyschev background functions and with pseudo-Voight line shapes with axial divergence asymmetry. A diamond anvil cell (DAC) was utilized to generate high pressure with the T301 stainless steel as the gasket, which was preindented to a 50-mm thickness. The high-pressure angle-dispersive XRD spectra were collected at beamline B2 of the Cornell High Energy Synchrotron Source (CHESS, Wilson Laboratory) at Cornell University for the WN with a monochromic wavelength of 0.4862 Å. One piece of the as-prepared samples was loaded into a gasketed diamond-anvil cell (DAC) with a 16:3:1 methanol/ethanol/water mixture as pressure-transmitting medium. Electrical resistivity of δ−WN was measured using a four-point probe system. The Vickers microhardness measurements were carried out using a microhardness tester (HV-1000ZDT) at a load ranging from 0.49 to 4.9 N for 15 s. The value of hardness could be calculated from the following formula

temperature of 2570 K using BN as nitrogen source. This may have been caused by low-energy activity of the nitrogen source. Even though high pressure could suppress as-synthesized products decomposing, regular high-energy activity of nitrogen source could not be used in HPHT method. So, finding a proper nitrogen source for use in HPHT method is the key factor for synthesizing δ−WN. Sodium azide was used as a high-energy activity nitrogen source in HPHT method.11 But the impure chemical element (Na) is easily introduced into products when sodium azide is used as nitrogen source in HPHT method. So, to find a new method to prepare the stoichiometric bulk δ−WN is not only helpful to verify its structure but also helpful to explore the mechanical properties for wide application. In this paper, high-quality bulk δ−WN was prepared via an untraditional method at HPHT using W2N3 and melamine as tungsten source and nitrogen source, respectively. The results of Rietveld refinements indicate that the as-synthesized δ−WN was close to ideal stoichiometry. The in situ XRD was carried out at high pressure in a diamond anvil cell (DAC). The results suggested that δ−WN was stable up to 24.8 GPa. The bulk modulus was obtained with a value of 373 ± 8.3 GPa which is consistent with that obtained from our theory calculation. It is amazing that the asymptotic HV of δ−WN is only 13.8 GPa. We first prove that W−W metallic bond and W−N ionic bond are mainly chemical bond in δ−WN based on the analysis of electron localization function (ELF), density of states (DOS), and Mulliken population. The reason for low hardness of δ−WN may be caused by the lack of strong W−N covalent bonds to form 3D network structure. This suggests that the hardness is not only determined by the conventional microstructural features but also determined by the nature of the bonding.

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HV = 1854.4P /d 2

(1)

where P is the load force and d is the average length of the two diagonals for the indent. Computational Methods. Density functional theory calculation within the CASTEP code was used.18 The exchange correlation energy was treated with the Perdew−Burke−Ernzerhof generalized gradient approximation (GGA-PBE),19 the Vanderbilt ultrasoft pseudopotential was used with the cutoff energy of 500 eV, and 17 × 17 × 17 Monkhorst−Pack scheme k-point sampling was used in the Brillouin zone. Electron localization function (ELF), density of states (DOS), and Mulliken population were carried out to estimate the type of W− N, W−W, and N−N bonds in δ−WN.

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RESULTS AND DISCUSSION The X-ray diffraction pattern of the as-synthesized sample verifies that the δ−WN is highly crystalline, as is shown in Figure 2. In general, crystalline structure refinement with X-ray diffraction data is an effective way to obtain accurate structural parameters. The Rietveld refinements using X-ray data were performed for δ−WN structure (Figure 2 inset) and the crystallographic data are shown in Table 11. This demonstrates

EXPERIMENTAL SECTION

Synthesis of Bulk WN. Powdery W2N3 and melamine powder (99.99%) were used as the starting materials. The details of the synthesis and purity process of W2N3 have been described in ref 8. W2N3 and melamine were chosen because high ionic activity cation W+ and high ionic activity N− can be obtained from the decomposition of W2N3 and melamine, respectively. Before the experiments, W2N3 and melamine powder were compacted into cylindrical pellets of 1 mm length and 4 mm diameter and of 0.75 mm length and 4 mm diameter, respectively. Sandwich-structured raw materials, in which the W2N3 pellet was surrounded by two melamine pellets on the top and bottom sides, respectively, were encapsulated in a h-BN container in order to minimize contamination during the synthesis process. Finally, the sample was synthesized in a cubic anvil HPHT apparatus (SPD-6 × 600) at 2073 K and 5.2 GPa for 15 min (as shown in Figure 1).

Figure 2. Rietveld refinement pattern of δ−WN. Solid dots: observed curve; red line: calculated curve; blue short vertical lines: Bragg positions; bottom line: difference curve. Crystal structure is displayed as inset.

Figure 1. Sample assembly for δ−WN synthesized under HPHT conditions: (a) high-pressure apparatus; (b) high-pressure cell for δ−WN growth. B

DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

modulus (B) is a significant elastic property correlated with the hardness. This obtained bulk modulus value is remarkably high, exceeding the bulk modulus of boron nitride (360 GPa) which has been widely used as superhard material.21 This exceptional property shows that δ−WN is a potential candidate as a novel hard or superhard material. The hardness of δ−WN was investigated in order to confirm the mechanical properties of the prepared δ−WN compact, and the results are shown in Figure 4. The hardness decreases with

Table 1. Structure Parameters from the Final Rietveld Refinement δ−WN cryst syst space group a, b (Å) c (Å) atom position W(1) B(1) residuals

hexagonal P6̅m2 2.90114(3) 2.82709(3) Wyckoff (x y z) 1a(0, 0, 0) 1d(1/3,2/3,1/2) Rwp: 0.0993 Rp: 0.0771 χ2: 1.476

that the prepared δ−WN has a hexagonal structure (space group P6̅m2) with lattice constants of a = 2.90114(3) Å and c = 2.82709(3) Å. It is worth noting that the lattice constants obtained in this work are close to the experiment value obtained by wang (a = 2.895 Å and c = 2.830 Å) and are a little different from the theoretical prediction value obtained by Qin (a = 2.861 Å and c = 2.905 Å).8,20 The result of successfully synthesized high-quality δ−WN indicates that our method is an effective route for synthesizing high-quality bulk δ−WN and melamine is a pure nitrogen source for introducing the nitrogen to the metal precursor. The in situ XRD patterns of δ−WN at various pressures up to 24.8 GPa have been collected and a few representative patterns are shown in Figure 3a. With increasing pressure, the diffraction patterns appear similar at each pressure point, and no new feature is observed, except for peaks shifting and broadening, indicating that the hexagonal structured δ−WN is stable up to 24.8 GPa. To calculate the modulus, the pressureinduced volume change was fitted to the Birch−Murnaghan equation of state

Figure 4. Vickers hardness of δ−WN measured at different applied loads. A typical Vickers indentation image at a load of 4.9 N is displayed as inset.

the increase of loading pressure. The hardness is converged to 13.8 GPa, which is much lower than the hardness value of WC (30 GPa) having lattice structure similar to δ−WN.22 This result indicates that δ−WN is not superhard material. And, this result also proved that δ−WN displayed a rather low hardness which is not as we want to get. Furthermore, even in the case that the crystal was not cracked at a load up to 4.9 N, indicating that δ−WN has high fracture toughness. The hardness indentation picture is shown in the inset of Figure 4. Generally speaking, more ionicity means more chance to have dislocation and high covalent structure means more brittle. So, the low hardness and high fracture toughness may be caused by no directional covalent bond between W atom and N. Hereinafter,

P = 3/2B0 [(V /V0)−7/3 − (V /V0)−5/3 ] × {1 − (3/4)(4 − B0 ′)[(V /V0)−2/3 − 1]}

(2)

where B0 is the bulk modulus and B0′ is its pressure derivative at the equilibrium volume V0. The bulk modulus was obtained with a value of 373 ± 8.3 GPa shown in Figure 3b when the derivative of the bulk modulus with respect to pressure. Bulk

Figure 3. (a) Selected XRD patterns of δ−WN at different pressures. (b) Pressure versus unit cell volume for δ−WN. Fitting these data with the Bircht−Murnaghan equation of state results in a bulk modulus of 373.2 GPa when B0 is fixed at the canonical value of 4. C

DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Elastic Constants, Bulk Modulus, Shear Modulus, and Vickers Hardness of Different Materials

a

compound

C11

C12

WC WN

712a 618.3(3.8)

241a 179.4(1.9)

C13

C33

C44

B (GPa)

G (GPa)

Hexp (GPa)

271.9(1.1)

977.5a 747.1(0.8)

305.1a 87.9(0.9)

439b 376.7(5)

282b 149.2(13.9)

30b 13.8

Ref 24. bRef 22

we try to explore the character of W−N bond to explain the hardness mechanism of δ−WN. The elastic constants, shear modulus, and bulk modulus are important parameters to understand many physical properties of solids materials including the hardness and compression properties. So, elastic constants, shear modulus, and bulk modulus were calculated based on the lattice constants parameter confirmed with Rietveld refinement. The theoretic results on elastic constants of δ−WN (P6̅m2) are listed in Table 2. The calculated elastic constants of δ−WN satisfy the Born−Huang’s stability criterion,23 indicating that δ−WN is mechanically stable. Among the elastic constants, C44 indicates the resistances of the crystal with respect to the shear strain at (100) plane.24 The value of C44 for δ−WN is 87 GPa which is much lower than the value of WC (305) as shown in Table 2. The low elastic constant C44, indicating deformation is easy to generate at (100) plane, may be caused by no covalent hybridization between W and N atom. Bulk modulus (B) and shear modulus (G) are two significant elastic properties correlated with the hardness, which were calculated to be 376.7 and 149.2 GPa, respectively, as shown in Table 2. The bulk modulus value is consistent with that obtained from our experiment. The obtained bulk modulus and shear modulus values are lower than those of tungsten carbide (HV ≈ 30.4), 439 and 282 GPa, respectively. The low shear modulus may also be caused by no directional covalence between W atom and N atom to resist plastic deformation and limit the creation and propagation of defects. Poisson ratio values are used to determine the ionic, covalent, or metallic nature of materials. Poisson’s ratio ν can be obtained from the formulas defined as ν = (3B − 2G)/(6B + 2G).25 The calculated Poisson’s ratio of δ−WN is ν = 0.32, which corresponds to ionic materials. The typical Poisson ratio values for covalent, ionic, and metallic are 0.1, 0.25, and 0.33, respectively.26 The value of the Poisson’s ratio (ν) of δ−WN is larger than 0.25. It may be caused by the ionic W−N bond and metallic W−W bond coexisting in δ−WN. Besides, for covalent and ionic materials, the typical relations between bulk and shear modulus are G ≈ 1.1B and G ≈ 0.6B, respectively.27 In δ−WN, the calculated value of G/B is 0.4, indicating that the ionic contribution to interatomic bonding between W atom and N atom is more suitable for δ−WN. So, it is conjectured that W−N ionic bond existed in δ−WN as major chemical bond. To our knowledge, electronic structure and chemical bonding are key factors to deeper understanding of the origin of hardness and elastic properties. Following, the density of states (DOS) and bond characteristic are calculated and analyzed here. The site projected and total DOS for δ−WN are shown in Figure 5, where the vertical line indicates Fermi level EF. It is clear that some bands are across the EF, indicating that δ−WN exhibits metallic behavior. On the other hand, the DOS profile near EF has sharp peaks and almost comes from the d state of W, indicating metallic behavior between the W atom and the W atom. Therefore, the W−W metallic bond would weaken the elastic properties and hardness of δ−WN. The total DOS almost comes from the d state of W, indicating

Figure 5. Total and partial density of states for δ−WN. The dotted line at zero is the Fermi energy level.

no covalent hybridization between W and N atom in these compounds. Moreover, the results of Mulliken populations calculation in this work indicate a large electronegativity difference between W (0.64 eV) and N (−0.64 eV). The quantity of electric charge transfer from tungsten atoms to nitrogen atoms indicates that the ioniciy character exists in the chemical bonding between W atoms and N atoms. It is believed that ionic bonding is not directly related to hardness, and the directional covalent bond effectively resists plastic deformation and limits the creation and propagation of defects, resulting in the excellent hardness of materials. The absence of covalent bonding between tungsten atoms and nitrogen atoms makes the structure, not 3D framework, easy to deform under shear stress and leads to the low hardness of δ−WN. To further gain more insight into the correlation between hardness and chemical bonding, the electron localization function (ELF) was calculated. The results of ELF for δ−WN (P6̅m2) are shown in Figure 6. There is no electron density located at the center of nearest two N atoms and no electron density between W atom and the nearest neighbor N atom, as shown in Figure 6a and b. So, it should form no covalence in W−N and N−N bonds. The electron density is clearly located at the center of W−W bonds in the W layers (Figure 6c), indicating metallic behavior between the W atom and the W atom, which is consistent with the analysis of DOS. So, the low hardness of δ−WN should be caused by W−W metallic bond and W−N ionic bond. D

DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from the National Natural Science Foundation of China (41572357). We acknowledge Dr. Yanchun Li for technical support with the high-pressure experiments at the B2 station of the CHESS.

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Figure 6. Electron localization function (ELF) of δ−WN, (a) and (b), the ELF (001) plane in the N layer and (111) plane of δ−WN, respectively; (c) the ELF W layer in (001) plane of δ−WN.

From the analysis given above, we can conclude that δ−WN should be a good conductor. So, electrical resistivity of δ−WN was measured using a four-point probe system. δ−WN shows excellent conductive characteristics with a low electrical resistivity of 9.964 × 10−7 Ω·m. It is reported that tungsten nitride has attracted considerable attention in modern very large scale integrated circuits (VLSI) as a diffusion barrier between Cu and Si substrate because of its low resistivity, easy patterning, and compatibility with conventional complementary metal-oxide-semiconductor (CMOS) technology.28,29 The low electrical resistivity of δ−WN suggests that δ−WN is a promising candidate for use as a diffusion barrier.

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CONCLUSIONS In summary, high-quality bulk δ−WN was successfully synthesized via an untraditional method. The result indicated that melamine is a pure nitrogen source for introducing the nitrogen to the metal precursor under HPHT. The results of Rietveld refinements indicated that the as-synthesized δ−WN was close to ideal stoichiometry. The in situ XRD was carried out at high pressure in a diamond anvil cell (DAC). The results suggested that δ−WN was stable up to 24.8 GPa. The bulk modulus was obtained with a value of 373 ± 8.3 GPa, which is consistent with that obtained from our theory calculation. It is worth nothing that the asymptotic HV of δ−WN is only 13.8 GPa, which is much lower than the previous theoretical predicted results. We proved that W−W metallic bond and W− N ionic bond are mainly chemical bond in δ−WN based on the analysis of ELF, DOS, and Mulliken population. This may be the reason for the relatively low hardness in δ−WN.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pinwen Zhu: 0000-0002-2909-9468 E

DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b03041 Inorg. Chem. XXXX, XXX, XXX−XXX