Effect of partial substitution of iron by tungsten on the crystal structure and electronic properties of WB3

Physica B 583 (2020) 412026

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Effect of partial substitution of iron by tungsten on the crystal structure and electronic properties of WB3 �n-Flores a, *, J. Rosas-Huerta a, M. Romero b, J.L. P�erez-Mazariego b, R. Go �mez b, J. J. Leo c a A. Arenas-Alatorre , R. Escamilla a b c

Instituto de Investigaciones en Materiales, Universidad Nacional Aut� onoma de M�exico, A.P. 70-360, Ciudad de M�exico, 04510, Mexico Facultad de Ciencias, Universidad Nacional Aut� onoma de M�exico, A. P. 70-399, Ciudad de M�exico, 04510, Mexico Instituto de Física, Universidad Nacional Aut� onoma de M�exico, A.P. 20-364, Ciudad de M�exico, 04510, Mexico

A R T I C L E I N F O

A B S T R A C T

Keywords: Tungsten triboride X-ray diffraction X-ray photoelectron spectroscopy M€ ossbauer spectroscopy Density functional theory

Polycrystalline samples of the W1-xFexB3 (x ¼ 0.00, 0.15) were synthesized by the first time by arc melting. The effect of the chemical pressure due to the substitution of tungsten by iron atoms on the crystal structure and electronic properties were studied by X-ray diffraction, electron microscopy, M€ ossbauer spectroscopy, X-ray photoelectron spectroscopy, and Density Functional Theory calculations. Rietveld refinement analysis indicate the presence of transition metal vacancies in the structure. The M€ ossbauer spectroscopy shows the presence of two non-equivalent crystallographic sites with different chemical environments, which are symptomatic of a charge density redistribution for one of the two iron sites and suggesting a vacancies formation mechanism. X-ray photoelectron spectroscopy and Density Functional Theory calculations show a similar density charge behavior for the tungsten atoms. Finally, an increment of electronic states at the Fermi level was predicted due to the partial iron substitution in the highest boride of tungsten.

1. Introduction Diamond and cubic boron nitride (cBN) are the hardest materials with a Vickers hardness around 90 GPa and 60 GPa, respectively. The high price of natural diamond, and difficulties associated with its extraction, together with the fact that carbon goes into solution when machining ferrous metals, are linked to the limited diamond application into the industry. For this reason, a method to produce synthetic ma­ terials was developed to find similar hardness and capabilities that could replace diamond, being cubic boron nitride one of the most promising materials [1]. Nevertheless, the extreme synthesis conditions for cBN (high temperature and high-pressure environments) made its produc­ tion quite expensive, restricting its scalable industrial application. Consequently, in recent years, investigation searching for high hardness new materials has taken place. It has been established that, in this new kind of compounds, direc­ tional covalent bonds are required to support both elastic and plastic deformations, although some small metallic or ionic bonding character is tolerable. Under this approach, light elements like nitrogen, boron, carbon, or even oxygen may combine with transition metals, to modify

the mechanical properties [2,3]. Several materials with high hardness have been obtained, some of them synthesized under extreme conditions while some others were obtained by conventional methods; for example: tungsten carbide (WC - experimental), Rhenium diboride (ReB2 experimental), Osmium diboride (OsB2 - experimental), iron tetraboride (FeB4 - experimental), manganese tetraboride (MnB4 - experimental), tungsten triboride (WB3 - experimental). Also, some high hardness materials have been predicted by means of theoretical calculations as for example: tantalum tetraboride (TaB4 - theoretical) and zirconium tri­ boride (ZrB3 - theoretical) [4–12]. In addition to tungsten carbide, the tungsten triboride compound (WB3) has been one of the most studied compounds due to the relatively simple synthesis method at ambient pressure conditions, compared with other hard materials synthesis. Moreover, the mechanical properties of the mentioned compound, such as the Bulk modulus (B ¼ 339 - 315 GPa), Shear modulus (G ¼ 266 - 126 GPa) and Vickers hardens (Hv ¼ 43 - 25 GPa) [13–15] are in the range of those reported values for WC (B ¼ 439 GPa, G ¼ 282 GPa, Hv ¼ 26 GPa) [16–18], which actually is one of the most explored and exploited ma­ terial based on a light atom and a transition metal. In the tungsten – boron binary composition, six different phases have

* Corresponding author. E-mail address: [email protected] (J. Le� on-Flores). https://doi.org/10.1016/j.physb.2020.412026 Received 6 September 2019; Received in revised form 21 December 2019; Accepted 20 January 2020 Available online 22 January 2020 0921-4526/© 2020 Elsevier B.V. All rights reserved.

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Fig. 1. (a) displays X-ray diffraction patterns for the different substitutions of iron and (b) Rietveld refinement of the W0.714Fe0.15B3 compound. Inset: crys­ tal structure.

been reported: α-WB, β-WB, W2B, WB2, W2B5 and WB4 (highest boride of tungsten), which in recent years has been established to be the WB3 phase [11,19–21]. Romans and Krug [11], are among the first who described the WB4 structure (1966); they determined a hexagonal crystal structure belonging to the P63/mmc space group with four for­ mula units (Z ¼ 4) per unit cell with two non-equivalent crystallographic Wyckoff positions for tungsten and boron atoms [W2 2c (1/3, 2/3, 1/4); W1 2b (0, 0, 1/4); B1 12i (x, 0, 0); B2 4f (1/3, 2/3, z)], describing a honeycomb-like networks of boron atoms, separated by planes formed by tungsten atoms, with the presence of boron dimmers between

tungsten layers. However, it was demonstrated that this kind of structure tends to present negative phonon modes, and consequently being ther­ modynamically unstable [21,22]. Considering both criteria, Zhang [21] determined a slightly modification from WB4 into WB3, implying the same hexagonal lattice belonging to the P63/mmc (No 194) space group with Z ¼ 4, but with the non-appearance of four boron atoms - all of them related with the boron dimmer -, B2 with Wyckoff position 4f (1/3, 2/3, z). More recently Zeiringer [23], Tao [14], Lech [24] and Gonzalez [15] have proposed the presence of tungsten vacancies, boron vacancies or even both to represent the highest boride of tungsten. To our 2

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knowledge, the works presented by Tao [14] and Lech [24], are the only two previous reports with X-ray photoelectron spectroscopy analysis (XPS) for the WB3 compound. However, Lech et al. present a survey spectrum of the WB3 compound as supplementary information while Tao et al. unfortunately made an inadequate fit to the tungsten W4f region because they linked the far away binding energies typically assigned to WO3 with those associated with tungsten borides. The present work describes the outcomes related to WB3 and W0.85Fe0.15B3 obtained for the first time by arc melting. To describe the influence of the substitution on the crystal lattice parameters were used €ssbauer and XPS X-ray diffraction and electron microscopy. Through Mo spectroscopies together with computational calculations based on Density Functional Theory (DFT) fundamental aspects over the elec­ tronic properties of the tungsten triboride compound are described.

Table 1 Structure parameters from the Rietveld refinement for the W0.86B3 and W0.71Fe0.15B3 compounds. Site

x/a

y/ b

z/c

W0.86B3

W0.71Fe0.15B3

W1/Fe1 2b

0

0

1/ 4

0.730 (2)

W2/Fe2 2c

1/3

2/ 3

1/ 4

1.00

B - 12i

0.3466(2) [W0.86B3] 0.3316(4) [W0.71Fe0.15B3]

0

0

1.00

0.692(1) [W1] 0.036(1) [Fe1] 0.964(2) [W2] 0.036(2) [Fe2] 1.00

3.26 10.36

3.24 9.66

χ2

Rb (%)

2. Materials and methods Tungsten monoboride (WB – Alfa Aesar 99%), iron monoboride (FeB – Alfa Aesar 99.5%) and Boron powder (B – Sigma Aldrich 99.999%) were mixed with a boron excess proportion ratio of 1:12 to avoid the formation of another tungsten boride phase [11,13]. The homogenous fine powder was compressed into pellets and melted with an electric arc furnace at a current value of 75 A under Argon atmosphere. The W1-xFexB3 samples were structurally characterized by X-Ray diffraction (XRD) together with Rietveld refinement method, scanning electron €ssbauer microscopy (SEM), transmission electron microscopy (TEM), Mo spectroscopy, X-Ray photoelectron spectroscopy (XPS) and computa­ tional calculations. X-ray diffraction was carried out with a Bruker D8 advance diffrac­ tometer at room temperature with a Cu Kα radiation in a range from 10� to 90� with steps of 0.02� in 2θ. The Rietveld refinement method was done by constrained-least-squares software [25]. SEM images were ac­ quired with an ultra-high-resolution electron microscope (JSM-7800F) at 15 kV of voltage acceleration with an �950 magnification. TEM im­ ages were obtained with a TEM-JEM2010 FEG electron microscope. Digital analysis of SEM and TEM images was done through the Digital €ssbauer spectra Micrograph software by GATAN. Room temperature Mo were recorded in transmission geometry with a constant acceleration spectrometer with a 57Co in an Rh matrix source and were fitted with the MossA software [26]. XPS measurements were performed in an ultra-high vacuum system Scanning XPS microprobe PHI 5000 Versaprobe-II, with an Al Kα X-ray source (hν ¼ 1486.6 eV), and a multichannel detector (MCD) analyzer. The surface of the samples was etched with 2.0 kV Arþ ion bombardment. The XPS spectra were ob­ tained at 45� to the normal surface in a constant pass energy mode, E0 ¼ 100 eV and 10 eV for survey surface and high-resolution narrow scan, respectively. Deconvolution adjustment for the XPS spectra was made by a least square process with the Spectral Data Processor software. The binding energy scale was referenced respect to adventitious C1s.

Occupancy

Bond length

W0.86B3 (Å)

W0.71Fe0.15B3 (Å)

W1/Fe1 - B W2/Fe2 - B B–B B–B

2.4013(2) 2.3253(2) 1.5965(2) 1.8036(1)

2.3428(2) 2.3526(2) 1.7250(1) 1.7521(2)

Table 2 Lattice parameters for the W0.86B3 and W0.71Fe0.15B3 compounds. Lattice parameter (Å)

W0.86B3

W0.71Fe0.15B3

Difference

aa (Exp) aa (DFT) ca (Exp) ca (DFT) ab (DFT) ac (Exp) cb (DFT) cc (Exp)

5.2037(2) 5.193 6.3411(3) 6.290 5.20 5.2012 6.34 6.3406

5.2022(1) 5.183 6.3383(2) 6.183 – – – –

0.37%

a b c

2.5%

0.8%

Lattice parameter values achieved at the present work. Ref [21] – where it was supposed a WB3 composition. Ref [14], Hexagonal lattice - P63/mmc (No 194).

for convergence were set to 1 � 10 6 eV/atom for the energy, 0.001 eV/Å for atomic force, 1 � 10 4 Å for ionic displacement and 0.02 GPa for stress. 4. Results and discussion 4.1. X-ray diffraction, SEM, and TEM Fig. 1(a) shows the X-Ray diffraction patterns of W1-xFexB3 com­ pounds. Small displacement of the peaks to low angles respect to the compound without Fe atoms is observed, indicating a modification of the lattice parameters. For the Rietveld analysis, were proposed a 50% of iron occupancies in the 2b(0, 0, 1/4) (denoted W1/Fe1) and 50% in the 2c(1/3, 2/3, 1/4) (denoted W2/Fe2) Wyckoff positions of the P63/mmc space group with Z ¼ 1; the corresponding Wyckoff position of B was 12i (x/a, 0,0). Metal transition vacancies in the 2b position (W1/Fe1) were considered according to the Rietveld refinement results, producing a modification into the stoichiometry of the compounds by W0.86B3 and W0.71Fe0.15B3 [14,15,23,24]. On the other hand, refinement of the B 12i position suggests the presence of different bond lengths B–B; indicating a distortion in the hexagonal boron arrangement at the W0.86B3 compound which decreases when iron replaces tungsten (W0.71Fe0.15B3), see Table 1. With that experimental information, DFT calculations were performed. Table 2 shows the experimental and theoretical lattice pa­ rameters compared with other reported experimental results. It is possible to observe that the experimental and theoretical lattice parameter values show a maximum difference of 2.5%; the percentage deviation of the values of this work from those reported by Zhang [21] and Tao [14] is 0.8%. It is possible to observe that the chemical pressure

3. Computational details Computational calculations were carried out within the Density Functional Theory (DFT) framework [27,28], as implemented in the Cambridge Serial Total Energy Package code [29], within the general­ ized gradient approximation with the exchange-correlation functional as it was proposed by Perdew and Wang [30] considering for boron, tungsten and iron atoms the 2s22p1, 5s25p65d46s1 and 3s23p63d64s2 as valence electrons, respectively. Tungsten substitution by iron atoms and vacancies were modeled through the virtual crystal approximation scheme [31,32]. The presence of tightly bound core electrons was rep­ resented by non-local norm-conserving pseudo-potentials of the Vanderbilt-type [33]. The wave functions were expanded in a plane-wave basis set with a cut-off energy of 550 eV with a k-point sampling inside the first Brillouin zone constructed using the Monkhorst-Pack scheme with 6 � 6 � 4 grids [29,34]. Tolerance criteria 3

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Fig. 2. (a) show the SEM image of the W0.86B3 sample putting into evidence de presence of β-rhombohedral boron phase. (b) displays the surface element identification.

promoted by the iron substitution inside the crystal structure of the compound has a significant effect over the c-direction while the a-di­ rection is almost unaltered. This fact is in concordance with the anisotropy associated with the hexagonal lattice, and it is possible to infer that the boron - metal transition bonding between layers is weaker than the metal-metal bonding interaction or boron – boron interaction. Moreover, this is well correlated with the fact that the honeycomb-like boron layers are composed of σ-bonding networks formed by the sp2 hybridization of the boron atoms [14,35]. Due to the excess of boron used in the synthesis of WB3 by arc melting, traces of boron with β-rhombohedral structure [36] were identified by SEM (Fig. 2) as previously has been reported [14,24]; however, by X-ray diffraction, this phase is not detected [37].

Fig. 3 shows the analyzed region by TEM, and it was indexed in concordance with the PDF card number 01-084-4051, which corre­ sponds to the P63/mmc (No 194) space group. The simulated crystal structure (inset Fig. 3) puts into evidence the arrangement of the W0.86B3 compound, with estimated cell parameters a ¼ 5.19 Å and c ¼ 6.29 Å. It is feasible to observe regions at the TEM image, where tungsten va­ cancies are perceptible, supporting the approximation made when Rietveld refinement was performed. 4.2. M€ ossbauer spectroscopy The M€ ossbauer spectrum of the W0.71Fe0.15B3 compound (Fig. 4) shows the contribution of three doublets with an iron population 4

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isomer shift is proportional to the difference between the electron density at the nuclei of the source and the absorber [38]: δ ¼ FðZÞ

ΔR � 2 ψ a ð0Þ R

�

ψ 2s ð0Þ

(1)

where ΔR is the change on the average nuclear radius between the excited and ground states of the nuclei, ψ 2a ð0Þ and ψ 2s ð0Þ are the electron densities at the absorber and the source nuclei; finally, F(Z) depends on the nuclear parameters. If the source is unchanged, a difference at the isomer shift between different sites of Fe in a complex implies different valence states among them, because the shielding effect on the 3s and 3p½ electrons changes (the n ¼ 1 and 2 shells are almost unaffected). Since Fe(II) has one more 3d electron, it increases the s and p½ electron densities at the absorber nucleus, increasing the δ value respect to Fe(III). The δ and ΔQ values of doublet 1 are typical for Fe(III) and, considering the above comments; doublet 2 must correspond to Fe(II) [38]. To be able to form the honeycomb layers, an sp2 hybridization (spxpy) of the B atoms must take place, living empty the pz orbital perpendicular to the B layer. Considering that the honey-comb boron layers are electron deficient, the electronic charge transferred to boron layers is completely delocalized as it was reported for borophene layers [35]. The presence of an itinerant pz electron promotes a mayor bonding between Fe2 atoms and boron, so it is highly probable that an electron charge transfer from Fe2 to boron takes place to gain stability for the structure. With this picture, the Fe1 atoms are limited to a transition metal – transition metal interactions (Fig. 5). Thus, according to the M€ ossbauer spectroscopy results, the apparent Fe2þ could be understood in terms of the idea of a charge transfer from Fe2 into boron [39–41]. Consequently, there is a higher probability of finding vacancies in the 2b than in the 2c Wyckoff position. In this sense, doublet 3, fitted with an isomer shift of 0.57 mm/s and a Δ ¼ 2.12 mm/s, gives an indirect insight into the presence of vacancies. Since the isomer shift value of 0.57 mm/s is closely like that fitted for doublet 2, it seems plausible that the same chemical state is preserved

Fig. 3. TEM from the W0.86B3 compound.

contribution of approximately 50% at the 2b site and 50% at the 2c site which supports the assumption made for the Rietveld refinement and the computational calculations. Doublet 1 was fitted with an isomer shift (δ) of 0.28 mm/s and with a quadrupole splitting (Δ) of 0.21 mm/s, the small value of the quadrupole splitting (Δ) are indicative of the poorappearance of an electric gradient field surrounding the iron nucleus and consequently being suggestive of a highly symmetric coordination environment around the iron atom (2b site). Doublet 2 was fitted with an isomer shift (δ) ¼ 0.55 mm/s and a quadrupole splitting (ΔQ) ¼ 1.16 mm/s. The great quadrupole splitting value for doublet 2 is due to the larger asymmetry of its environment at the 2c site related to the presence of an electric gradient field. The fact that the δ value of doublet 2 is almost twice the corresponding one to doublet 1 implies a different valence state of Fe for each site because the

Fig. 4. M€ ossbauer spectrum of the W0.71Fe0.15B3 compound. Three doublets contributions are observed, each one related to every iron environment surrounding in the P63/mmc crystal structure. 5

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Fig. 5. Schematic structure environment for the iron atom at 2c site sensed by M€ ossbauer spectroscopy. Under this arrangement, there is a weaker bonding interaction between Fe1 and boron atoms compared with Fe2 atoms with boron. The doublet 3 contribution at the 2c site: Fe2 is related to the appearance of vacancies at the 2b site (dotted ellipse).

35.15–35.69 eV for W 4f7/2 core level (Table 4) which are in range of the reported values for tungsten oxide [43–45] that must not be associated with the tungsten triboride compound as it was done previously by Tao [14]. The presence of a small amount of tungsten oxide it is supported by the identified oxygen contribution localized at 530 eV, commonly associated with the presence of W-O bonding interaction [45] and complemented by the fact that there is a small amount of unreacted tungsten due to the tungsten vacancies. The main contribution of from W 4f core level is located at binding energy values of 33.48 eV (W 4f5/2 orbitals) and 31.37 eV (W 4f7/2 orbitals) for W0.86B3 compound. These values are slightly lower than those reported for WC (W 4f7/2 31.5–31.8 eV) [46,47] and tungsten – boron composite materials B4C–W2B5 (W 4f7/2 31.7 eV) [44] but are within the range for the WB2 compound with P63/mmc space group (W4f7/2 31.45 eV) [48]. Besides, the 31.37 eV for the W 4f7/2 core level is higher than the binding energy values commonly associated with metallic tungsten (W 4f7/2 core level with a binding energy of 31 eV) [42,43]. All this together is indicative of the presence of a charge transfer mechanism from tungsten into boron atoms in a similar way as it was elucidated before by M€ ossbauer spec­ troscopy for the iron atoms. The B 1s orbital deconvolution shows three main contributions. The low energies around 187 eV are assigned to the presence of the β boron rhombohedral phase [49]. The regions above the 190 eV are related to the small contribution to boron oxides phases [44] due to the presence of oxygen and the boron excess needed to the tungsten boride synthesis. The 188 eV region are associated with the borophene like layer contri­ bution as it was reported for similar arrangements [48,50] however as it could be observed there is a small shift into lower energy when the iron substitution was done, again, related with the promotion of the charge transfer from transition metal into the boron layer. Nevertheless, since the XPS spectroscopy has not the same accuracy €ssbauer, the charge transfer hypothesis as a nuclear technique like Mo was reinforced by Mulliken population analysis and the Electron Localization Function (ELF) computed for the W0.86B3 compound. The Mulliken charge obtained for tungsten atoms was 1.32 e while for boron was 0.44 e . These results put into evidence a polarized charge behavior between boron and tungsten atoms. The population analysis using ELF (Fig. 7) shows how the zero values (colored bar scale) near to

Table 3 M€ ossbauer hyperfine parameters from the W0.71Fe0.15B3 compound. Doublet 1 (2b Site: Fe1) Doublet 2 (2c Site: Fe2) Doublet 3 (2c Site: Fe2)

δ (mm/s)

Δ (mm/s)

% site

Γ (mm/s)

0.28(1) 0.55(3) 0.57(1)

0.21(2) 1.16(1) 2.12(2)

50.3(6) 36.4(1) 13.3(6)

0.26(1) 0.38(2) 0.40(1)

Isomershift (δ) relative to metallic iron.

between the two iron atoms. Nevertheless, it is observed that quadru­ pole splitting value for doublet 3 is large enough, compared with the Δ ¼ 1.16 mm/s fitted for doublet 2, being indicative of a highly asym­ metric environment surrounding due the presence of vacancies at the 2b site in the neighborhood to the Fe2 atoms enhancing the contribution to the electric gradient field existing around the 2c site. Moreover, it is observed (Table 3) a 13.3% population contribution of the doublet 3, which is related to the presence of vacancies. This percent population site is in good agreement with the 14% of vacancies estimated by X-ray diffraction. 4.3. X-ray photoelectron spectroscopy The survey spectra of the W0.86B3 and W0.71Fe0.15B3 compounds is presented in Fig. 6(a). It seems the presence of carbon, oxygen, and nitrogen elements, similar to what it was presented by Lech [24] as supplementary information for the reported survey spectra of the highest boride of tungsten. Fig. 6(b) shows the high-resolution spectra for the Fe 2p. The spectrum comprises a simple spin-orbit doublet with narrow, symmetric components in which splitting is about 13.4 eV. The Fe 2p1/2 and Fe 2p3/2 core levels are localized at 720.99 and 707.62 eV, respectively. These values are consistent with the reported for elemental iron [42]. Fig. 6(c)-(d) and (e)-(f) displays the high-resolution spectra for the B 1s and W4f. It is observed two different spin-orbital splitting contributions for tungsten and seems that the minor contributions are those binding en­ ergies between 37.35 and 37.73 eV for W 4f5/2 core level and 6

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Fig. 6. (a) show the XPS survey of the W0.86B3 and W0.71Fe0.15B3 compounds. At (b) it is shown the Fe 2p core level region for the W0.71Fe0.15B3 compound. (c) – (d) shows the high resolution XPS spectra of the B 1s and W 4f regions of the W0.86B3. Finally, (e) – (f) shows the B 1s and W 4f core-level regions for W0.71Fe0.15B3. Table 4 XPS core level binding energies. The estimated standard deviations for the binding energies and spin-orbit splitting values were �0.05 eV. Core level region

W0.86B3

W0.71Fe0.15B3

BE (eV)

ΔE (eV)

BE (eV)

ΔE (eV)

Fe 2p1/2 Fe 2p3/2 W 4f5/2 W 4f7/2 W 4f5/2 (oxide) W 4f7/2 (oxide) B 1s (β – Boron)

– – 33.48 31.37 37.35 35.15 186.91

–

720.99 707.62 33.73 31.60 37.73 35.69 187.85

13.37

B 1s (Borophene like layer) B 1s (Suboxide) B 1s (Oxide) O 1s (W–O) O 1s (B–O)

188.47 190.04 – 530.70 532.02

2.11 2.20 – – –

188.30 190.12 192.04 530.88 532.44

2.13 2.04 – – –

the tungsten atoms are regions of highly delocalization electrons, while the colored bar scale was increased the electrons are highly localized. The mayor localized character of the ELF was between boron – boron interaction and consequently supporting the evidence of σ -bonding ar­ rangements produced by the sp2 hybridization between the boron atoms. As it was reported previously, the presence of distorted sp2 bonding is observed [14] (white circles in Fig. 7) altering the bond distances between boron atoms. This effect was supported by the different B–B bond lengths estimated by the Rietveld refinement (Table 2). This bonding mechanism could be extrapolated into the highest boride of tungsten doped by iron, because the proposal of

Fig. 7. It is shown the presence of distorted sp2 bonding (white circles – online version) around the 2c site for the ELF in the W0.86B3 compound.

distorted bonding, correlates well with the presence of a high value of the quadrupole splitting in the 2c sites (Table 3) and is indicative of an asymmetric surrounding around iron atom denoted as Fe2. Furthermore, it was observed an increase at the assigned W 4f7/2 core level region from 31.37 to 31.60 eV, as iron was substituted by tungsten indicating that iron substitutions promote the charge transfer into the boron layers.

7

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Fig. 8. (a) and (b) shows the total and partial density of states of the W0.71Fe0.15B3 and W0.86B3 compounds. (c) displays the comparing of the total density of states for both compounds.

4.4. Density of states

were achieved. The hypothesis on the iron site occupancy made from the €ssbauer analysis agrees with the results Rietveld refinement and the Mo obtained by DFT calculations. The lattice parameters behavior as a function of iron substitution indicates that the chemical pressure effect over the crystal structure tends to modify it, mainly along with the cdirection respect to the a-direction, being indicative of a strong bonding €ssbauer spectros­ interaction along the a-direction. Finally, through Mo copy, XPS and DFT calculations the mechanism of the density charge distribution between layers inside the structure was determined and pointed out as the responsible of the transition metal vacancies on the highest boride of tungsten compound.

Fig. 8(a) and (b) show the total and partial density of states of the W0.86B3 and W0.71Fe0.15B3 compounds. Due to the substitution of tungsten by iron atoms, it is observed an increase in the total density of states of W0.86B3 from 1.67 to 5.3 states/eV associated with the increase of Fe 3d states, which suggest an increment of the charge carriers. A common characteristic of the transition metal - boron compounds is the appearance of a sharp valley around the Fermi energy [51], and it has been established that this valley formation has three possible ori­ gins; one is related to some ionic character between the components; another is the presence of a p - d orbitals interaction; an alternative process is related to the hybridization between p orbitals, as it was evidenced for the AlB2 compound where there are not interacting d or­ bitals; and finally, a possible resonance between d – d orbitals [41]. The fact that the main profile of the sharp valley is unchanged by the iron substitution clarifies that the principal mechanism to the valley forma­ tion in the highest tungsten boride compound is the sp2 hybridization at the borophene-like layer. Nevertheless, the slight modification derived from the iron substitution, as it could be seen in the partial DOS (Fig. 8 (a)), is a consequence of a partial degeneracy breaking between the B2p and the W5d orbitals near the 2 eV region derived from a competition between the B 2p orbitals which interact with the transition metal d orbitals against a metal-metal interaction. (W-Fe)

CRediT authorship contribution statement � n-Flores: Conceptualization, Methodology, Writing - original J. Leo draft, Investigation, Formal analysis, Writing - review & editing. J. Rosas-Huerta: Formal analysis, Writing - review & editing. M. Romero: �rez-Mazariego: Formal analysis, Writing - review & editing. J.L. Pe � mez: Investigation, Formal analysis, Writing - review & editing. R. Go Formal analysis, Writing - review & editing. J.A. Arenas-Alatorre: Investigation, Formal analysis, Writing - review & editing. R. Escamilla: Supervision, Conceptualization, Methodology, Formal analysis, Writing - review & editing. Acknowledgments

5. Conclusions

This research was partially supported by the UNAM-DGAPA-PAPIIT �n-Flores and J. programs IN109718/30, IN115219, and IN114416. J. Leo Rosas-Huerta would like to thank CONACYT for the received fellowship during this work. For providing technical help, the author’s thanks

In this work were studied the crystal structure and electronic prop­ erties of the tungsten triboride doped with iron. According to the microstructural results, stoichiometries of W0.86B3 and W0.71Fe0.15B3 8

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Physica B: Physics of Condensed Matter 583 (2020) 412026

Adriana Tejeda-Cruz, L. Huerta, Samuel Tehuacanero C., A. Vivas, A. Pompa. Calculations were performed using resources from the Super­ computing Center DGTIC-UNAM.

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