Synthesis, Hardness, and Electronic Properties of Stoichiometric VN

Article pubs.acs.org/crystal

Synthesis, Hardness, and Electronic Properties of Stoichiometric VN and CrN Shanmin Wang,*,†,‡,# Xiaohui Yu,§ Jianzhong Zhang,∥ Liping Wang,† Kurt Leinenweber,⊥ Duanwei He,‡ and Yusheng Zhao*,† †

HiPSEC & Physics Department, University of Nevada, Las Vegas, Nevada 89154, United States Institute of Atomic & Molecular Physics, Sichuan University, Chengdu 610065, China § National Lab for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ∥ Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ⊥ School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States ‡

S Supporting Information *

ABSTRACT: We report synthesis of single-crystal VN and CrN through high-pressure ionexchange reaction routes. The final products are stoichiometric and have crystallite sizes in the range of 50−120 μm. We also prepared VN and TiN crystals using high-pressure sintering of nitride powders. On the basis of single-crystal indentation testing, the determined asymptotic Vickers hardness for TiN, VN, and CrN is 18 (1), 10 (1), and 16 (1) GPa, respectively. The relatively low hardness in VN indicates that the metallic bonding prevails due to the overfilled metallic σ bonds, although the cation−anion covalent hybridization in this compound is much stronger than that in TiN and CrN. All three nitrides are intrinsically excellent metals at ambient pressure. In particular, VN exhibits superconducting transition at Tc ≈ 7.8 K, which is slightly lower than the reported values for nitrogen-deficient or crystallinedisordered samples due to unsuppressed “spin fluctuation” in the well-crystallized stoichiometric VN. The magnetostructural transition in CrN correlates with a metal−metal transition at TN = 240(5) K and is accompanied by a ∼40% drop in electrical resistivity. In addition, more detailed electronic properties are presented with new insights into these nitrides.

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1000 °C, the x values in CrNx decrease with increasing temperature.28 Above 1050 °C, instead of cubic CrNx, the final synthesis product is mainly hexagonal Cr2N, a more stable compound in the binary Cr−N system.28 Similar situations also occur in the Ti−N and V−N systems.16,29,30 Although a prolonged nitridation process has successfully been used for growing large crystals of CrNx and VNx,29,31 the final products are largely substoichiometric with x < 1. As a result, most reported nitrides are substoichiometric and poorly crystallized in forms of thin films. In fact, such crystalline defects have profound influences on the physical properties of these nitrides. The disorder-driven superconductor-insulator transition, for example, has previously been reported in TiNx.9,11 The nonstoichiometry-induced elastic softening in TiNx has also been found.12,16 In the case of CrNx, crystalline defects have led to controversial reports of magnetic, structural, and electronic properties.24,28,32,33 Thus, experimental data based on stoichiometric and single-crystal nitrides are needed to accurately determine their intrinsic properties. Under high pressure (P), a number of novel nitrogen-rich TM nitrides have recently been discovered in the Zr−N,34,35 Hf−N,34 Ta−N,36,37 and noble metal nitride systems,38−44

INTRODUCTION Incorporation of nitrogen atoms into the lattice of transition metals (TM) forms a large number of nitride compounds.1−4 Of particular interest are the early 3d transition-metal mononitrides, which have attracted considerable attention because of their fundamental importance in condensed-matter physics and computational physics.5−15 This family of nitrides also has excellent mechanical, electronic, and magnetic properties for a wide variety of technological applications,16−18 such as semiconductor (e.g., ScN),19 superconductors (e.g., VN and TiN),11,20 and magnetic materials (e.g., CrN, MnN, and CoN).17,21,22 Depending on the number of 3d electrons in the constituent metals, the nitrides crystallize in several different structures, including rock-salt (e.g., CrN),11,19,20 tetragonal (e.g., MnN),21 and zinc-blende (e.g., FeN and CoN) structures.21,22 Among them, the rock-salt TiN, VN, and CrN are the most studied and technologically useful nitrides.5−18,23,24 At atmospheric pressure, most TM nitrides are prepared by using traditional synthetic routes such as ammonolysis or nitridation of metals and their compounds (e.g., Cr2S3),1,25 vapor deposition,26 and epitaxial growth methods.24 Because of high−temperature (T) degassing, the equilibrium nitrogen concentration (x) in TMNx is strongly dependent on the synthesis temperature. The stoichiometric CrN, for example, is only stable below 800 °C;24,27 at higher temperatures up to © XXXX American Chemical Society

Received: September 10, 2015 Revised: November 1, 2015

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DOI: 10.1021/acs.cgd.5b01312 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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suggesting that pressure can effectively suppress the high-T degassing and promote the involvement of d-electrons in chemical bonding.45 Most recently, we have synthesized a series of novel N-rich nitrides (e.g., W2N3, W3N4, and MoN2) through ion-exchange reactions at pressures of 3.5−5 GPa.27,32,46,47 Using prolonged heating up to 1600 °C, the single-crystal δ-MoN and cubic γ-MoN were also prepared with a grain size of 50−80 μm.48 The determined asymptotic Vickers hardness for single-crystal δ-MoN is close to 30 GPa, which is so far the hardest metal nitride. In this work, we extended this methodology to VN and CrN and have successfully synthesized single crystals for both nitrides. Large single-crystal VN and TiN were also prepared using a high-P sintering. With such improved specimen, we further determined their intrinsic hardness and electronic properties.

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EXPERIMENTAL SECTION

High-purity powders of hexagonal boron nitride (hBN) (>99.9%, ∼50 μm) and alkali-metal oxides of Na2CrO4 (>99.5%, ∼50 μm), NaVO3 (>99.5%, ∼50 μm), and KVO3 (>99.5%, ∼50 μm) were used as starting materials for high-P synthesis. Na2CrO4 was obtained by dehydration of commercially available Na2CrO4·2H2O in a muffle furnace at 200 °C for 12 h. High P−T synthesis was carried out in a large-volume cubic press with experimental detail described in ref 49. To obtain phase-pure nitride powders, the starting materials in the molar ratios Na2CrO4: BN = 1:2 and NaVO3 (or KVO3): BN = 3:5 were homogeneously mixed and compacted into cylindrical pellets (12 mm diameter and 10 mm length) for the synthesis of CrN and VN, respectively. In each experimental run, the pellet was contained in a molybdenum capsule to prevent potential contamination. At a target pressure of 3.5−5 GPa, the starting reactants were quickly heated to 1300 °C within 3−5 min (∼300 °C/min) and then soaked for 20 min before quenching to room temperature. In order to grow large single crystals, the alkali-metal oxide was sandwiched between two hBN pellets, instead of homogeneously mixed reactants. In these experiments, the temperature was slowly increased to 1600 °C at a rate of ∼5 °C/min to facilitate crystal nucleation and growth, followed by quenching to room temperature. To remove the reaction byproduct, NaBO2, and unreacted starting reactants, the experimental run products were washed with nitric acid and hydrofluoric acid for CrN and VN, respectively, followed by washing in distilled water and drying in an oven at 75 °C. In addition, using a Kawai-type high-P apparatus,50 we also hot-pressed the VN powders from high-P synthesis and as-purchased TiN powders to grow large crystals at 6−8 GPa and 1800 °C for 45 min. More detailed experimental procedures have been described elsewhere.27,49,50 The final products were characterized by X-ray diffraction (XRD) with Cu Kα radiation, optical microscopy, field emission scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoemission spectroscopy (XPS). Low-T fourprobe resistivity measurement was performed on sintered polycrystalline bulk samples, using a commercially available Quantum Design PPMS. The Vickers hardness was measured from single-crystal indentation under different loads of 25, 50, 100, and 200 g by using a Micromet-2103 hardness tester (Buehler, USA). Under each applied load, the measurement was performed with a dwelling time of 15 s and was repeated 5−10 times to obtain statistically improved averages. Before measurements, the specimen was mounted on a SiO2 or an Al2O3 substrate using the epoxy resin, and the mirror-quality surfaces were prepared for the measurement.

Figure 1. XRD patterns of the purified nitrides collected at room temperature using a copper Kα radiation. (a) CrN. (b) VN with V2O3 impurity. (c) VN. Phase-pure nitrides in (a) and (c) were synthesized from homogeneously mixed starting reactants at 5 GPa and 1300 °C for 20 min. Single-crystal VN in (b) was synthesized using a sandwiched sample assembly at 3.5 GPa and prolonged heating (see Experimental Section for detail). Inset is a polyhedral view of structure for these rock-salt TM nitrides including CrN, VN, and TiN.

analysis for VN in Supporting Information Figure S4). As discussed in our previous work (ref 27), CrN is formed from two-step reactions between Na2CrO4 and BN, Na 2CrO4 + BN = NaCrO2 + NaBO2 + 1/2N2

(1)

NaCrO2 + BN = CrN + NaBO2

(2)

In the first step (reaction 1), the intermediate phase, NaCrO2, starts to form at relatively low temperatures of ∼500 °C. The ion-exchange reaction 2 is the second and final step for the formation of CrN at higher temperatures. The detailed description of the reactions 1 and 2 can be found in ref 27. Similarly, the formation of VN is subject to a two-step reaction, given by 3NaVO3 + 2BN = 3NaVO2 + B2O3 + N2

(3)

NaVO2 + BN = VN + NaBO2

(4)

The intermediate phase NaVO2 (refer to PDF #44-0342) is identified in our XRD measurement, when the molar ratio NaVO3:BN = 3:2 was used in the experiment at 3.5 GPa and 1000 °C (see Supporting Information Figures S1 and S2). The reaction 4 can simply be viewed as an ion-exchange process between V3+ and B3+. The similar reactions 3 and 4 also occur between KVO3 and BN (Supporting Information Figure S2). Preparation of large single-crystal VN and CrN was initially attempted using homogeneously mixed starting reactants. Because of widespread nucleation within the sample volume, the crystal sizes in the final run products are limited to only several microns. To reduce the number of nucleation sites, a sandwich-like sample assembly was employed for preparing large crystals (see Experimental Section). As will be discussed later, substantially larger crystals were obtained from this experimental procedure. Figure 1b shows a typical XRD pattern of VN with V2O3 as the only impurity phase, obtained from the sandwich assembly and prolonged heating of ∼5 h. V2O3 may

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RESULTS AND DISCUSSION Figure 1 shows X-ray diffraction (XRD) patterns of the final run products. At ambient conditions, both CrN and VN adopt the rock-salt crystal structure. In Figure 1a,c, phase-pure CrN and VN were obtained from homogeneously mixed starting reactants at 5 GPa and 1300 °C for 20 min (see elemental B



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form from the intermediated phase NaVO2 through high-T decomposition (i.e., 2NaVO2 = V2O3 + Na2O) as reported previously.51 Compared with VN, CrN shows more resistance to corrosion in nitric acid. Therefore, the phase-pure CrN crystals were obtained after a two-day treatment with nitric acid at room temperature. The refined ambient lattice parameters, a, for high-P synthesized VNx and CrNx are 4.1361(2) and 4.1513(2) Å, respectively (also see Supporting Information Table S1). According to the x−a relationships as shown in Figure 2,27,29,33 the determined x for both nitrides is very close to a

Figure 3. Typical SEM and optical images of CrN. (a−d) CrN crystals synthesized using a sandwiched sample assembly at 3.5 GPa (see Experimental Section). (e) CrN powders synthesized from homogeneously mixed starting reactants at 5 GPa and 1300 °C for 20 min. (f) Rod crystals obtained from re-sintering of the CrN powders in (e) at 14 GPa and ∼700 °C for 1 h. On the basis of single crystals in (d), Vickers hardness of CrN was measured under loads of 0.245 and 0.49 N, and the corresponding indentations are numbered with ‘1’ and ‘2’, respectively.

Figure 2. Lattice parameter as a function of nitrogen concentration for TMNx. (a) VNx (e − ref 29). (b) CrNx (f − ref 28, g − ref 33, and h − ref 8). Also plotted in (a) is an enlarged portion to show detail. The determined x value for the as−purchased VNx is ∼0.991.

the re-sintered VN crystals exhibit uniform geometries with a grain size of 10−50 μm, which is more suitable for hardness measurement. Figure 4h shows typical indentation pyramids on single crystals under different loads. The similar high-P sintering method was also used to synthesize large crystals of TiN at 6 GPa and 1800 °C for 45 min. Figure 5a shows aspurchased starting TiN powders, and the grain size is less than 3 μm. The synthesized TiN crystals have a moderate grain size of 10−50 μm (Figure 5b,c), similar to those of the re-sintered VN crystals (Figure 4g). Figure 5d presents the indentation pyramids on TiNx single crystals. It is worthwhile to mention that attempts to synthesize nitrides of group 4 elements (e.g., Ti and Zr) were unsuccessful at pressures up to 5 GPa, using the similar ion-exchange reactions between Na2XO3 (X = Ti and Zr) and hBN. Instead, the final products are metal oxides (e.g., ZrO2). Because of the limited crystallite size, the determination of crystallographic dependence of hardness for these nitrides is still challenging. Future work along this direction is warranted to explore more detail of hardness on the basis of larger single-crystal samples. Figure 6 summarizes the Vickers hardness, HV, for all three nitrides as a function of applied load, obtained from microindentation on single crystals with randomly oriented crystallographic directions (see Figures 3d, 4h, and 5d). At the loads exceeding 0.49 N, the HV values of TiN, VN, and CrN approach asymptotic values of 18(1), 10(1), and 16(1) GPa. Different from hard (e.g., Moissanite)52 and superhard materials (e.g., BC2N),53 the HV values in all three nitrides are leveled off at much smaller load of 0.49−1 N, above which the plastic deformation starts to prevail.54 For TiN and CrN, the hardness is comparable to that of other mononitrides including NbN, ZrN, and HfN with an HV of 15−20 GPa.55 For VN, it is so far the softest metal mononitride with a low

unity (i.e., stoichiometric VN and CrN). In a previous report on single-crystal CrNx (ref 31), the measured lattice parameter is 4.1467 Å, corresponding to a sub-stoichiometric composition of x ≈ 0.990. Besides, the measured a and x values for aspurchased VNx are a = 4.1338 (2) Å and x ≈ 0.991, respectively (Supporting Information Figure S3). Clearly, the high-P ionexchange reaction is a more effective route to synthesize stoichiometric nitrides. Typical SEM and optical images of CrN single crystals are presented in Figure 3a−c, indicating that the grain size is in the range of 50−120 μm. The crystals are morphologically similar to the Cr−N octahedron depicted in Figure 1, suggesting that the crystal growth in CrN is preferred along the [111] crystallographic direction. Figure 3d shows two typical indentation pyramids on a CrN crystal under loads of 0.245 and 0.49 N. Because of the limited crystallite size, the entire crystal was severely cracked at loads exceeding 0.49 N. Figure 3e shows CrN powders with small grain size of 1−2 μm synthesized from a homogeneous mixture of Na2CrO4 and BN. After resintering of the CrN powders at 14 GPa and ∼700 °C for 1 h, the crystals display rod shape morphology as shown in Figure 3f, likely because recrystallization occurs in the P−T region where high-P orthorhombic phase is thermodynamically more stable than cubic phase.32 From reactions 3 and 4, the brownish-yellow crystals of VN form with crystallite size of 50−100 μm, as shown in Figure 4a−f. The coexisting blue-black colored crystals are V2O3 (Figure 4a,e), which was confirmed by XRD measurement in Figure 1b. Figure 4e,f shows platelike crystals of VN, which is morphologically distinct from the CrN crystals (octahedral shape), an indication that they have different preferred crystallographic directions for crystal growth. In Figure 4g, C



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Figure 6. Vickers hardness of single-crystal VN, CrN, and TiN as a function of load.

because the valence electron concentration in VN (i.e., 9) is higher than the critical value of 8.4 as pointed out by Jhi et al.14 However, based on nano-indentation on thin-film coatings, Helmersson et al. previously reported a high value of 16.2 GPa for VN,57 which would substantially overestimate the hardness value, because of the influence of substrate and other extrinsic complications such as lattice strain and defects.54,58 In addition, the determined HV values in this work for these nitrides are greatly different from those of recent first-principles calculations (i.e., 24, 14, and 1.2 GPa for TiN, VN, and CrN, respectively),59 probably due to the theoretical difficulties in properly modeling such correlated 3d-electron nitride system. Figure 7 presents the low-T resistivity measurements on VN and CrN. VN is an excellent metal with a low electrical

Figure 4. Typical optical images of VN. (a) Bulk crystals (brownishyellow color). Also formed blue-black crystals are V2O3 impurity phase (see XRD in Figure 1b). (b−d) Selected crystals. Panels (e) and (f) platelike crystals. (g) Polycrystalline bulk sample prepared by resintering of the high-P synthesized VN powders at 8 GPa and 1800 °C for 45 min. The observed black areas in (g) are polished mirror surfaces on single-crystal VN. (h) Single-crystal indentations for VN. Indentation pyramids numbered with ‘1’ ‘2’, ‘3’, and ‘4’ correspond to the loads of 0.245, 0.49, 0.98, and 1.96 N, respectively.

Figure 7. Low-T electrical resistivity measurements. (a) VN. (b) CrN. The cubic-orthorhombic CrN transition happens at TN = 240(5) K. The experimental data of both cubic and orthorhombic phases are fitted to a conventional resistivity eq 5.

resistivity, ρ, of ∼0.06 mΩ·cm (Figure 7a), which agrees well with a previous report.60 The superconducting transition in this stoichiometric sample starts at Tc ≈ 7.8 K with a transition width of ∼2.5 K, and the resistivity is saturated at zero below ∼5.1 K. The determined Tc (∼7.8 K) is slightly lower than frequently reported values of 8.5−9.2 K in sub-stoichiometric and/or disordered VNx samples synthesized using traditional routes.60,61 As reported previously, the “spin fluctuation” coexists and competes with superconductivity in VNx,61 and a certain degree of crystalline defects would suppress the “spin fluctuation” and hence prompt a higher Tc. The suppression of

Figure 5. Typical SEM and optical microscope photographs of TiN. (a) As-purchased TiN powders. (b) Sintered polycrystalline TiN at 6 GPa and 1800 °C for 45 min. The observed black areas in (b) are polished mirror surfaces. (c) An enlarged portion of the sample to show detail. (d) Vickers hardness measurement based on single-crystal TiN (also see Figure 4h).

hardness of HV ≈ 10 GPa, close to that of GaN (∼10.8 GPa)56 and standard steel block.52 This unusual low hardness in VN may be associated with the overfilling of metallic σ bond, D



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“spin fluctuation” can also be achieved at high pressure as revealed in ref 20. As a neighboring nitride of vanadium, CrN exhibits intriguing magnetostructural transition at low temperature or high pressure.8,17,32 Coupling with the antiferromagnetic transition (Figure 7b), the cubic-to-orthorhombic structural transition in stoichiometric CrN happens at the Néel temperature, TN, of 240(5) K with a ∼40% drop in electrical resistivity. Because of the sensitivity of phase stability to the nitrogen deficiency x in CrNx, the previously reported TN is quite inconsistent (i.e., in a range of 276−300 K) and tends to increase as the x decreases.8,17,28,62 Evidently, the low value of TN = 240(5) K in this work suggests that the nitrogen concentration in our sample is close to ideal stoichiometry with x = 1. In fact, the nearly stoichiometric CrN thin films have previously been prepared using the reactive sputtering method and the observed TN is around 250 K,33 which is very close to our measurement. In Figure 7b, the electrical resistivity of CrN decreases gradually as temperature decreases (i.e., having a positive temperature coefficient of resistivity, TCR). For both cubic and orthorhombic phases, the electrical resistivity is comparable to that of VN. The ρ−T data can be fitted to the conventional resistivity equation for a metal, ρ(T) = ρ0 + AT n

Figure 8. Observed 2p and N1s XPS spectra for CrN and VN. (a−d) 2p XPS peaks of metal cations. For comparison, the reported spectra of TiN (ref 63) and ScN (ref 64) are also plotted. (e) N1s peaks (see refs 63 and 64 for ScN and TiN). For VN, a shoulder peak on the high-energy side (∼399.7 eV) may arise from unreacted hBN. The cyan and green dotted lines represent the experimental data for CrN and VN, respectively. The solid lines correspond to the fitted data. All these nitrides have the rock-salt structure.

(5)

where ρ0 is the residual resistivity and A is a material-specific constant. The fitted n values for cubic and orthorhombic CrN are 1 and 4, respectively, suggesting that the electrical resistance in both phases originates from the phonon-electron interaction. Thus, we conclude that cubic and orthorhombic CrN are intrinsic metals and the associated electronic transition is a metal-metal transition across the TN. Because of these unique magnetic and metallic properties for CrN, the 3d3 electrons in Cr are partially delocalized and both itinerant and localized electrons coexist. In contrast, the unpaired 3d electrons in TiN and VN are largely delocalized and are served as charge carriers. To gain more insights into electronic properties, we performed core-level XPS measurement on CrN and VN. In Figure 8, we show the observed M2p (M = Cr and V) and N1s spectra (see Supporting Information Figure S5 for full XPS spectra), and the obtained binding energies are listed in Table 1. For comparison, the reported M2p spectra for TiN and ScN are also plotted.63,64 As shown in Figure 8a−d, all M2p spectra are split into two components: M2p1/2 and M2p3/2, due to the strong spin-orbit coupling of 2p states.65 Each of the M2p1/2 and M2p3/2 components has a doublet structure composed of a high-energy main peak and a low-energy satellite peak. For the corresponding metals, only singlet peaks can be observed,66 indicating that the M2p1/2 and M2p3/2 doublets arise from the anion-cation hybridization. On the basis of the well-established metal−ligand cluster model,65,67,68 the main and satellite peaks are attributed to the multiple configurations of dn+1L̲ and dn (where L̲ denotes a ligand hole), respectively. Accordingly, the energy separation, ΔE, between the two peaks is closely associated with the cation−anion hybridization effect (i.e., the p−d hybridization),65 given by ΔE ≈ 2Veff

Table 1. Summary of the Obtained M3p, M3s, M2p, and N1s Binding Energy (B.E.) and Their Peak Width (Full with at Half Maximum, FWHM) for Nitrides of MN (M = V and Cr)a metal V66

VN M = V or Cr N 1s M3p M3sb M2p3/2b M2p1/2b

B.E. (eV)

FWHM (eV)

397.3 40.3 65.6 69.0 514.2 517.0 521.8 524.5

2.1 5.5 3.4 5.1 2.5 2.6 2.8 3.0

B.E. (eV) 37.2 66.3 512.1 519.8

metal Cr66

CrN B.E. (eV)

FWHM (eV)

396.4 43.2 74.6 78.6 575.0 576.8 584.6 586.2

2.3 4.5 4.0 4.5 2.7 2.7 3.3 3.5

B.E. (eV) 42.2 74.1 574.1 583.8

a

The values of pure metals Cr and V are also added for comparison. The peak widths of main peak and the associated satellite of M2p3/2 line in both VN and CrN are considerably sharpened when compared with those of M2p1/2 line (also see Figure 8), because of the exchange coupling between 2p holes and 3d electrons (see ref 67 for detail). A similar situation is also observed between the main peak and satellite for M3s line (Figure 9), mainly due to the charge-transfer effect (see refs 67 and 65). b

pure ionic compound.64 In addition, the Veff values of these nitrides are substantially smaller than those of their oxide (e.g., ∼6.5 eV for V2O3)65 and halide counterparts (e.g., ∼3.3 eV for MnF2),68 implying the weak p−d hybridization in these nitrides. At an initial glance, both Cr 2p1/2 and Cr 2p3/2 lines in Figure 8a are singlets, contrary to what one might expect. A careful analysis shows that both peaks are significantly broadened, and each of them can be fit as a doublet. The best fit of the Cr2p data gives a relatively small Veff of ∼0.9 eV (see Table 1), even smaller than that of VN and TiN, probably

(6)

From Figure 8b−d, the p−d hybridization strength, Veff, for VN, TiN, and ScN are determined to be ∼1.4, 1.0, and 0 eV, respectively (also see Table 1), which is consistent with the increasing electropositivity of metal ions from V to Sc. The determined Veff for semiconducting ScN also indicates it is a E



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CONCLUSIONS In summary, we synthesized large crystals of VN and CrN through the high-P ion-exchange reaction routes using sandwiched sample assemblies. The final run products are stoichiometric with grain sizes of 50−120 μm. The crystal growth of CrN is preferred along the [111] crystallographic direction, which led to morphologically uniform and octahedron-like crystals. Because of the absence of such preferred direction for crystal growth, VN crystallized in different shapes. The large crystals of VN and TiN were also synthesized through high P−T resintering. The determined asymptotic HV values for TiN, VN, and CrN based on single-crystal indentation are 18(1), 10(1), and 16(1) GPa, respectively. Using a core-level photoemission technique, the detailed electronic structures for the early 3d transition-metal nitrides were explored in terms of a metal−ligand cluster model. The determined anion−cation covalent hybridization strength decreases systematically in the order of VN, TiN, CrN, and ScN. Interestingly, the softest VN exhibits the strongest hybridization, mainly due to the over-filled metallic σ bond. Because of the absence of crystalline defect, the spin fluctuation in the stoichiometric VN is unsuppressed and, hence, gives a low Tc of ∼7.8 K. In addition, both cubic and orthorhombic CrN are intrinsic metals. The measured Néel temperature for stoichiometric CrN is TN = 240(5) K, and the associated electronic transition is metal-to-metal with a ∼ 40% drop in electrical resistivity.

due to the magnetic properties of CrN with partially localized 3d electrons.17 Obviously, the p−d hybridization strength decreases systematically in the order of VN, TiN, CrN, and ScN. Interestingly, the same trend is also seen for the N1s lines (Figure 8e), implying that the N1s binding energy increases with increasing hybridization strength (i.e., the Veff value). The relatively high Veff value in VN indicates that the hybridized p− d bonding is highly covalent; yet VN is so far the softest metal nitride with a hardness of HV ≈ 10 GPa (see Figure 6). Such paradox is presumably associated with the complicated bonding in the TM nitrides that involves metallic, ionic, and covalent bonds. For VN, the metallic bonding should be overwhelming due to the over-filled metallic σ bond as mentioned above.14 Compared with the M2p spectra, the splitting of M3p line is very small and almost negligible (see Figure 9), owing to the

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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.cgd.5b01312. Observed XRD patterns for the recovered products before and after purification, EDX, and full XPS patterns (PDF)

Figure 9. Observed M3s and M3p XPS spectra. (a) CrN. (b) VN. Peaks and background (B.G.) of the observed spectra are deconvoluted using least-squares methods. V3s shows an unusual stronger intensity of satellite on the high-energy side than that of the main peak on the low-energy side. The similar situation also occurs in the V2p spectra (see Figure 8b), likely due to the energy-level inversion between the multiplets of dn+1L̲ and dn (see ref 68).

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AUTHOR INFORMATION

Corresponding Authors

*(S.W.) E-mail: [email protected]. *(Y.Z.) E-mail: [email protected]. Present Address #

weak spin-orbit interaction in 3p orbitals.65,67,68 Instead, the M3p line is largely associated with the 3p−3d Coulomb multiple coupling between a 3p hole and 3d electrons.65 For the M3s spectrum, it primarily originates from the 3s−3d exchange interaction. In this regard, the preformed 3s core holes (s̲) have net spin−up (↑) or spin−down (↓) moments to magnetically interact with the outer and unpaired 3d electrons,65 leading to the splitting of M3s peak, as shown in Figure 9. The energy separation of splitting in CrN is ∼4 eV and is slightly larger than that in VN (∼3.4 eV), indicating stronger intra-atomic exchange interaction in CrN. To the best of our knowledge, most reported studies of XPS have been concentrated on TM oxides and halides with insulating band structures and localized 3d electrons,65,67,68 giving rise to the simplest multiplets of dn and dn+1L̲. However, situations in these nitrides are considerably complicated, either because the unpaired 3d electrons are partially (e.g., CrN) or completely delocalized (e.g., TiN), or because the 3d orbitals are unoccupied (e.g., ScN). Further theoretical work along this direction is warranted to understand their detailed electronic structures.

Chemical & Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by UNLV High Pressure Science and Engineering Center (HiPSEC), which is a DOE NNSA Center of Excellence operated under Cooperative Agreement DEFC52-06NA27684, and UNLV startup funding to Y.Z. This work was partially supported by the China 973 Program and NNSF of China (Grant Nos. 2011CB808205, 11427810, and 51472171). We also thank M. Chen for help on the sample synthesis and XPS measurement.

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REFERENCES

(1) Oyama, S. T. Chemistry of Transition Metal Carbides and Nitrides; Springer: Berlin, 1996. (2) Pierson, H. O., In Handbook of Refractory Carbides & Nitrides: Properties, Characteristics, Processing and Applications; William Andrew: Burlington, 1996.

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