Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten

Article pubs.acs.org/cm

Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten Nitrides Shanmin Wang,†,‡ Xiaohui Yu,‡ Zhijun Lin,‡,§ Ruifeng Zhang,∥ Duanwei He,†,* Jiaqian Qin,⊥ Jinlong Zhu,‡ Jiantao Han,‡ Lin Wang,⊥ Ho-kwang Mao,⊥ Jianzhong Zhang,‡,* and Yusheng Zhao‡,#,* †

Institute of Atomic & Molecular Physics, Sichuan University, Chengdu 610065, P. R. China LANSCE Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Geophysical Laboratory, Carnegie Institution of Washington, NW Washington, D.C. 20015, United States ∥ Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ⊥ Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan # HiPSEC, University of Nevada, Las Vegas, Nevada 89154, United States ‡

S Supporting Information *

ABSTRACT: Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W−N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W2N3. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W2N3, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production. KEYWORDS: tungsten nitride, high-pressure synthesis, ion exchange

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INTRODUCTION Transition metal nitrides have recently attracted considerable interest because of their fundamental importance in condensedmatter and computational physics and because of their unique mechanical, catalytic, optical, electronic, and magnetic properties for technological applications.1−8 At atmospheric pressure, transition metal nitrides are commonly prepared through heating metals in a stream of ammonia (NH3) of nitrogen gas, but reactions are often incomplete, leaving behind unreacted metals or low nitrogen concentration in the final product.3−5 In addition, there are only a few cases in which the reaction between metals and nitrogen gas leads to the formation of nitrides.8 In the binary tungsten−nitrogen (W−N) system, nitrides are typically prepared by nitridation of tungsten metal via nonequilibrium processes using NH3 flow. Depending on the starting forms of W (powder or thin film) and flow rate of NH3, several nitrides were obtained with different stoichiometric compositions and crystal structures. Two polymorphs of WN, for example, were reported using powder W, and they are structurally isotypic with NaCl and WC structures. Other © 2012 American Chemical Society

NaCl-structured WxNy, such as W2N and W3N2, were also reported.4 Nitrides prepared from tungsten thin films include hexagonal W2N, W5N4, W2.54N4, and W5N8 and rhombohedral W7N6 and W2.36N2.4 An alternative approach, although not commonly adopted, is to evaporate W from filaments through heating to temperatures above 2373 K in atmospheres of N2;4 this route led to the formation of WN2. A thorough overview of the W−N system can be found in ref 4. Tungsten nitrides, however, are intrinsically difficult to prepare, primarily because the incorporation of nitrogen into the tungsten lattice is thermodynamically unfavorable at atmospheric pressure. As a result, most materials in the binary W−N system are produced as thin films with poor crystalinity, and for the majority of known nitrides the crystal structures are either not determined or remain a controversy. Received: May 16, 2012 Revised: July 25, 2012 Published: July 25, 2012 3023

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Figure 1. XRD patterns of nitrides in the binary system W−N. The observed (green crosses) and calculated (solid orange lines) profiles for h-W2N3 (a) and r-W2N3 (b), synthesized at 5 GPa and temperatures of 1480 and 880 K, respectively, and purified by removing byproduct NaBO2 (see Experimental Section); c-W3N4 (c) and δ-WN (d), prepared from h-W2N3 at 5 GPa and 2273 K and 2570 K, respectively. The curves in the cyan color represent the difference between the observed and calculated profiles. Insets are crystal structures of those nitrides. The blue tick marks correspond to the peak positions.

Table 1. Summary of Structural Parameters for h-W2N3, r-W2N3, c-W3N4, and δ-WN Determined by XRD Refinements h-W2N3 space group a0, c0 (Å) ρ (g cm−3) W positions

P63/mmc (No. 194) 2.890(3), 15.286(2) 12.31 4f, (1/3, 2/3, 0.840)

N positions

4f, (1/3, 2/3, 0.070) 2c, (1/3, 2/3, 1/4) 2.163(3) 8.64

dW−N (Å) RP (%) a

r-W2N3 R3 (No. 146) 2.889(7), 15.291(5) 12.29 3a, (0, 0, 0) 3a, (0, 0, 0.831)a 3a, (0, 0, 0.249) 3a, (0, 0, 0.416) 2.090(4), 2.104(4), 2.111(5) 6.82

c-W3N4

δ-WN

Pm3m ̅ (No. 221) 4.125(6) 14.37 3c, (1/2, 1/2, 0)

P6̅2m (No. 187) 2.895(5), 2.830(4) 16.00 1a, (0, 0, 0)

1b, (1/2, 1/2, 1/2) 3d, (1/2, 0, 0) 2.063(4) 8.78

1d, (1/3, 2/3, 1/2) 2.190(3) 7.96

S.O.F. = 0.3333.

High pressure and temperature (P−T) synthesis has been demonstrated to be an effective route for the discovery of novel transition metal nitrides because the oxidation states of the metals tend to vary with pressure. Recent examples of novel nitrides include Th3P4-type Zr3N4 and Hf3N4,7 noble metals nitrides (Pt, Ir, and Os),9−11 orthorhombic η-Ta2N3,12 and hexagonal Re2N and Re3N,13 all with promising elastic bulk moduli and hardness. On a negative note, the required pressures for these syntheses are relatively high, in a range of 11−50 GPa, which is beyond the current technological capability for massive and industrial-scale production. To date, however, no novel nitrides have been reported in the W−N system using the high P−T synthesis route. Very recently, first-principles calculations predicted the existence of two high-pressure polymorphs of WN2 with bulk modulus around 410 GPa and Vickers hardness of 36 GPa.2 This combined situation indicates that there may be a wide-open window of opportunities for new findings and breakthroughs. In this work, we explore the nitride synthesis in the binary W−N system under high P−T conditions. A novel approach involving solid-state reaction between Na2WO4 and BN is developed, which led to the discovery of nitrogen-rich W2N3

with hexagonal and rhombohedral structures at 5 GPa and 880−1770 K. We also successfully prepared the cubic W3N4 and WC-structured WN from the nitrogen degassing of W2N3 and W3N4 at 5 GPa and 2273−2570 K. We determined the crystal structures of the four nitrides using the Rietveld method and studied their elastic properties by both compression experiments and first-principles calculations. Experimental details can be seen in the Methods section.

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RESULTS AND DISCUSSION Figure 1a shows an X-ray diffraction (XRD) pattern of the powder sample synthesized at 5 GPa and 1480 K for 20 min. Because the decomposition of the run product results in the formation of W3N4 as discussed below, this nitride must be nitrogen-rich with x > 1.33 in WNx. The nitrogen content was further examined from chemical analysis and was found to be in the range of x = 1.3−1.8 (see Experimental Section). All diffraction peaks in Figure 1a can be indexed by a hexagonal unit cell with a0 = 2.890 (3) Å and c0 = 15.289 (2) Å. The XRD pattern resembles those of W4.6N4 or W5N4, having a hexagonal crystal structure.4 The space group P63/mmc (No. 3024

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hexagonal or rhombohedral W2N3 under atmospheric pressure at temperature up to 1273 K, indicating that the solid-state reaction 1 is thermodynamically favored under high pressure. Figure 1c shows a representative XRD pattern of the run product formed at 5 GPa and 2273 K for 20−25 min using purified h-W2N3 as starting material. All diffraction lines of this profile match a cubic, P-centered lattice with a0 = 4.125 (6) Å. This crystal lattice is commonly associated with space group Pm3m ̅ (No. 221), which is allowed for this unknown nitride. The refinement results evidently show that the Wyckoff site 3c (0, 1/2, 1/2) is the only position for W atoms. As discussed below, the higher-temperature degassing of this leads to the formation of a known nitride with a molar ratio W:N = 1. Accordingly, N anions are allowed to occupy the positions 1b (1/2, 1/2, 1/2) and 3d (1/2, 0, 0). This structural resolution strictly limits the molar ratio of W:N to be 3:4 and hence a stoichiometric composition of W3N4 (c-W3N4). The final refinement results are summarized in Table 1. When temperature is further increased to 2570 K at 5 GPa, a hexagonal phase is formed and it is structurally isotypic with WC (Figure 1d).5 The refined lattice parameters (see Table 1) agree well with previously reported results for δ-WN,4 confirming a molar ratio of W:N = 1:1 for this hexagonal nitride. Clearly, c-W3N4 and δWN are formed by the following degassing reactions under high P−T conditions:

194), which is commonly adopted in layer-structured materials (e.g., graphite, hBN, and MoS2), is therefore applied to this novel nitride for the structural refinements. On the basis of the density measurement, the Wyckoff site 4f (or 4e) is the only possible position for W in the crystal structure (see more information in the Experimental Section). Our refinement indicates that the best fit is achieved in 4f (1/3, 2/3, z) and z = 0.840. In fact, trigonal prismatic coordination {XN6} (X = Mo, W) for X atoms was preferably adopted in most cases.14−16 Accordingly, the unit cell contains six N atoms in the Wyckoff positions of 4f and 2c. This crystal structure resolution indicates a molar ratio of W:N = 2:3, which is a novel nitride in the system W−N and is hereafter referred as h-W2N3. The measured density on purified product is ∼11.80 g/cm3 (see Experimental section) and is within 4% of the calculated X-ray density of 12.31 g/cm3 for h-W2N3, which provides further support for the stoichiometric composition determined by chemical analysis and crystallographic consideration. The final refinement results are summarized in Table 1 with crystal structure of h-W2N3 depicted in Figure 1a. Figure 1b shows an XRD pattern for the run product synthesized at 5 GPa and 880 K for 45 min. Note that this is a profile after removing the diffraction peaks of hBN; a full and uncorrected XRD profile can be found in Supporting Information Figure S1. All those diffraction peaks cannot be attributed to any of the known tungsten nitrides. On the other hand, they can be indexed by a rhombohedral unit cell with a0 = 2.889 (7) Å and c0 = 15.291 (5) Å, which is structurally similar to those reported for ternary nitrides of LiMoN2, LiWN2, and CrWN2, all with the same space group of R3 (No. 146).14−16 Another common feature in this family of ternary nitrides is that both W and Mo atoms sit in the trigonal prismatic coordination, while Li or Cr is positioned at octahedral interstices. In some cases, such as LiMoN2, antisite Mo atoms are also found in an octahedral hole.14 Energy dispersive X-ray (EDX) analysis measurement shows that this rhombohedral nitride only contains two elements of W and N (see Supporting Information Figure S2). Further treatment of this nitride at 5 GPa and 1223 K (starting with the purified sample) leads to the conversion into h-W2N3, indicating that it is a low-temperature phase of h-W2N3; it is hereafter denoted as r-W2N3. Similar to ternary nitrides, r-W2N3 is composed of two alternating coordination variants: trigonal prismatic W1 and octahedral W2, as shown in Figure 1b. Because trigonal prismatic coordination is preferable for W atoms, the site occupancy factor (S.O.F.) of 1/3 is assumed for octahedral W2 to satisfy the molar ratio of W:N = 2:3. Detailed refinement results are shown in Figure 1b and Table 1. For as-synthesized run product of both h-W2N3 and r-W2N3, NaBO2 was identified by powder XRD as a reaction byproduct, indicating that they are formed through the following high P−T solid-state reaction, 2Na 2WO4 + 4BN = W2N3 + 4NaBO2 +

1 N2 2

3W2N3 = 2W3N4 +

W3N4 = 3WN +

1 N2 2

1 N2 2

(2)

(3)

Typical scanning electron microscope (SEM) images of hW2N3, r-W2N3, c-W3N4, and δ-WN are presented in Figure 2. Apparently, the observed layered, plate-shaped h- and r-W2N3 (Figure 2a,b) originate from the laminated nature of their

(1)

The unwanted NaBO2 can readily be removed from the assynthesized run product to obtain pure-phase W2N3 (see Experimental Section). Throughout this article, the purified rand h-W2N3 refer to the reaction products from which NaBO2 has been removed. Because the starting materials (Na2WO4 and BN) contain W6+ and N3−, the reaction can be simply viewed as an ion exchange process between W and B.17 The synthesis route of reaction 1, however, failed in producing either

Figure 2. SEM images of W−N compounds. (a) h-W2N3 synthesized at 5 GPa and 1480 K for 20 min; (b) r-W2N3 with hBN impurity synthesized at 5 GPa and 880 K for 45 min; in both (a) and (b), the as-synthesized samples were purified by removing the byproduct NaBO2; (c) c-W3N4 prepared from the decomposition of h-W2N3 at 5 GPa and 2273 K for 25 min; (d) mixed phases of c-W3N4 and δ-WN prepared from the decomposition of h-W2N3 at 5 GPa and 2570 K for 10 min. 3025

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Compared with other transition-metal nitrides of the same metal:nitrogen molar ratio, the measured K0 for h-W2N3 is slightly higher than that of η-Ta2N3 (327 GPa).20 The assintered h-W2N3 sample has 4% porosity based on X-ray density and the Vickers hardness is Hv(0.5) = 14 GPa, which is comparable to the reported value of Hv(0.5) = 16 GPa for ηTa2N3 with 10% porosity. Unlike traditional layer-structured materials (e.g., hBN21) exhibiting extremely high interlayer compressibility, our measurements show that the c-axis is elastically stiffer than the a-axis in h-W2N3 (see Supporting Information Figure S4). It implies that the N−W−N sandwich layers are linked through strong covalent N−N bonds, rather than the van der Waals forces. A similar phenomenon has been predicted by theoretical simulations on WN2.2 For c-W3N4, the determined K0 is 50% and 45% higher, respectively, than those of Th3P4-type c-Zr3N4 (217−250 GPa)7,22 and c-Hf3N4 (260 GPa).7 This superior behavior can be attributed to the bonding nature in that the nearest-neighbor cation−anion bond length in c-W3N4 (2.063 (4) Å, see Table 1) is noticeably shorter than those in c-Zr3N4 and c-Hf3N4 (2.17−2.49 Å). In order to underline the stiffness or hardness of the newly synthesized cW3N4, the shear modulus of c-W3N4 is calculated by an ab initio method and is found to be 183 GPa, which is also higher than that of c-Zr3N4 (163 GPa).22 It is well recognized that the shear modulus is a better indicator of hardness in the design of novel hard/superhard materials because it ultimately measures the plastic deformation under indentations.19,23 Because c-Zr3N4 has been reported to have a hardness of 36−40 GPa,8 it can be expected that c-W3N4 has a similar hardness or perhaps is in a superhard regime, which is commonly defined as Hv > 40 GPa. Last but not least, we find that the predicted bulk modulus for δ-WN (see Table 3) is more than ~6% higher than that of PtN (372 GPa).9 These comparisons show that the novel nitrides we unveiled in the W−N system visibly stand out among transition-metal nitrides with superior mechanical properties. According to the Born−Huang criterion,24 c-W3N4 and δWN are mechanically stable, whereas h-W2N3 is mechanically unstable. The discrepancy between the computation and observations for h-W2N3 may partly be attributed to the vacancy or oxygen stabilization effect as revealed in η-Ta2N3.20 To gain insights into bonding characteristics of those nitrides, the electronic density of states (DOS) of h-W2N3, c-W3N4, and δ-WN were computed and are illustrated in Figure 3. All three phases show similar metallic bonding features with finite DOS

respective crystal structures (Figure 1a,b). In striking contrast to the morphology observed in h-W2N3, rather uniform crystal size and equant crystal shape are observed in c-W3N4, as shown in Figure 2c. This crystal morphology is further confirmed in mixed phases of c-W3N4 and δ-WN (Figure 2d). The observed microstructures are consistent with the crystal structures determined in this work. The elastic properties obtained from high-pressure synchrotron experiments and first-principles calculations are summarized in Tables 2 and 3 (selected synchrotron diffraction Table 2. Summary of Bulk Modulus K0 (in GPa) and Its Pressure Derivative K′ for W−N Compounds Obtained from Both High-Pressure Compression Experiments and FirstPrinciples Calculations material

measured K0/K′

h-W2N3 r-W2N3 c-W3N4 δ-WN

331(12)/4.0(5) 226(20)/4.0(9) 376(15)/4.0(5)

calculated K0/K′

368(2)/4.2(2) 396(3)/4.1(1)

Table 3. Single-Crystal Elastic Constants, the Voigt Bulk Modulus K0, and Shear Modulus G for c-W3N4 and δ-WN (all in GPa)a material

c11

c-W3N4 δ-WN c-BN

789 641 786

c33

c12

731

144 214 172

c13

c44

267

99 112 445

c66

K0

G

213

368 396 376

183 172 390

a

Previous theoretical results for c-BN (ref 18) are also included for comparison.

patterns are shown in Supporting Information Figure S1). The determined bulk moduli (K0) from both the experiment (376(15) GPa) and the calculation (368(2) GPa) for c-W3N4 are comparable to that of cubic boron nitride (c-BN).18,19 For δ-WN, first-principles calculations predict an even superior elastic behavior with K0 = 396(3) GPa. For h-W2N3, although it is elastically more compressible than c-W3N4 and δ-WN (see Supporting Information Figure S3), the experimentally determined K0 (331(12) GPa) is still within ∼10% that of cBN. These exceptional properties indicate that they are potential candidates of novel hard or superhard materials.

Figure 3. Total and partial electronic density of state for (a) h-W2N3, (b) c-W3N4, and (c) δ-WN. The vertical dashed lines indicate the Fermi levels. 3026

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indexed by a hexagonal structure with space group P63/mmc (No. 194) and lattice parameters of a = 2.890 (3) Å and c = 15.286 (2) Å. The measured density for well-sintered h-W2N3 is ∼11.80 g/cm3. Because XRD intensities are dominated by W atoms in any tungsten nitrides and because the N atoms contribute less than 11% to the total Bragg scattering power and 13% to density, the N atoms could be ignored and only W positions were included for the initial refinement. In order to determine the atomic positions of W atoms, Wyckoff site 24l (x, y, z), the highest allowable symmetry for the space group of P63/mmc, was first designated for W atoms; the calculated density, however, is 66.27 g/cm3, approximately six times higher than the measured value. Thus, because the multiplicity factor of 4f or 4e is 1/6 that of 24l, the Wychoff site 4f or 4e is the only possible position for W atoms. Microstructure of the purified samples was characterized by a field emission scanning electron microscope (SEM) (Inspect, The Netherland). The microscope was operated at an electron accelerating voltage of 15 kV. Prior to the SEM analysis, the sample surface was sputtered with a thin gold layer to avoid charging effects. Energy dispersive X-ray (EDX) was used for compositional analyses. Vickers hardness was measured on well-sintered h-W2N3 samples with a load of 0.5 kg and dwelling time of 15 s, denoted as Hv(0.5), by using a hardness tester Micromet-2103 (Buehler, USA). High-P synchrotron X-ray experiments using diamond-anvil cell (DAC) techniques were performed at the HPCAT 16BM-D beamline (High Pressure Collaborative Access Team) of the Advanced Photon Source (Argonne National Laboratory, Argonne, IL). The powder nitrides were loaded into the sample hole in the rhenium gaskets with helium as pressure-transmitting medium in all experimental runs. The samples in DAC were compressed up to 15−50 GPa at room temperature. Monochromic X-ray beam with wavelengths of 0.398137 Å, 0.406620 Å, and 0.413363 Å were used, and the MarCCD detector was employed to collect the data. The collected diffraction data were analyzed by integrating 2D images as a function of 2θ using the program Fit2D27 to obtain conventional, one-dimensional diffraction profiles. The pressures in all of the experiments were determined by the ruby scale.28 Ab Initio Calculations. First-principles calculations were performed using the VASP (Vienna Ab Initio Simulation Package) code29 with the generalized-gradient approximation proposed by Perdew and Wang for an exchange-correlation functional. The integration in the Brillouin zone was employed using the Monkhorst-Pack scheme (9 × 9 × 9), an energy cutoff of 600 eV, and a tetrahedron method with Blöchl corrections for the energy calculation and Gaussian smearing for the stress calculations. The conjugate gradient method was used for the relaxation of structural parameters. The first-principles calculations of single-crystal elastic constants conducted here for cubic and hexagonal crystals are similar to previous studies of other materials to which we refer for further details.30

at the Fermi level (EF), which originates mostly from 5d electrons of W and 2p electrons of N and is consistent with our measurements that h-W2N3 and c-W3N4 are both electrical conductors. In addition, there is a strong hybridization between N 2p and W 5d states in all three nitrides as revealed by the appearance of “pseudogap” underneath the Fermi level, indicating a mixture of covalent and ionic contribution to the bonding between W and N atoms in a similar manner as that shown in the ReNx conpounds.18

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CONCLUSION In summary, we formulated a new solid-state reaction route for high P−T synthesis of transition metal nitrides. The employment of this technique successfully produced two novel nitrogen-rich nitrides, h-W2N3 and r-W2N3, in the binary W− N system. The discovery of W2N3 also offers a new and direct route for high-pressure synthesis of cubic W3N4 and hexagonal WN through nitrogen degassing at elevated temperature. The synthesis of all four nitrides is at a moderate pressure of 5 GPa, which is so far the lowest among the pressures reported for the synthesis of novel nitrides in the systems Zr−N, Hf−N, Ta−N, Re−N, Os−N, Ir−N, and Pt−N and is technologically suitable for massive and industrial-scale production. The final products of h-W2N3, c-W3N4, and δ-WN are phase-pure and wellcrystallized in bulk forms. In particular, they exhibit elastic properties rivaling or even surpassing cubic boron nitride (cBN) and other novel transition metal nitrides. We have extended the method to Mo−N and Cr−N systems and found a number of novel nitrogen-rich nitrides; the structure refinements and property characterization are currently in progress and will be published elsewhere.

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EXPERIMENTAL SECTION

Synthesis, Purification, and Characterization of the Samples. High P−T synthetic experiments were carried out in a multianvil cubic press. Pressure and temperature generation and calibration were described in our previous work.25 Na2WO4 and hBN were used as starting materials in high-pressure experiments. Na2WO4 was chosen because the cation W6+ favors the formation of nitrogen-rich nitrides. Before the experiments, homogeneous Na2WO4 and hBN (in a molar ratio of 1:2) powder mixtures were compacted into cylindrical pellets of 10 mm length and 12 mm diameter. A Mo capsule was used to prevent possible contamination. Experimental details are illustrated elsewhere.25 In order to obtain phase-pure tungsten nitrides, the recovered sample was first grounded into powder and washed in distilled water to remove the byproduct NaBO2, then treated in high-concentration HNO3 and NaOH solutions at 333 and 363 K for 1 h, respectively, to get rid of possible indissoluble impurities, and finished by second water washing and drying in a high-T oven (