WC-type Structure Research Articles

On the growth kinetics, texture, microstructure, and mechanical properties of tungsten carbonitride deposited by chemical vapor deposition

Tungsten carbonitride [W(C,N)] was deposited on cemented carbide substrates by chemical vapor deposition (CVD) in a hot-wall reactor using tungsten hexafluoride (WF6), acetonitrile (CH3CN), and hydrogen (H2) as precursors. Tungsten carbides and nitrides with a hexagonal δ-WC type structure are generally difficult to obtain by CVD. Here, it was found that the combination of WF6 and CH3CN precursors enabled the deposition of W(C,N) coatings with a δ-WC type structure and columnar grains. A process window as a function of the deposition temperature and precursor partial pressures was determined to establish the conditions for the deposition of such coatings. Scanning electron microscopy, x-ray diffraction, electron backscatter diffraction, and elastic recoil detection analysis were used for the investigation of the coating thickness, microstructure, texture, and composition. From the investigation of the kinetics, it was concluded that the growth was mainly controlled by surface kinetics with an apparent activation energy of 77 kJ/mol, yielding an excellent step coverage. The partial reaction orders of the reactants together with their influence on the microstructure and coating composition was further used to gain a deeper understanding of the growth mechanism. Within the process window, the microstructure and the texture of the W(C,N) coatings could be tailored by the process parameters, enabling microstructural engineering with tuning of the mechanical properties of the W(C,N) coatings. The nanoindentation hardness (36.6–45.7 GPa) and elastic modulus (564–761 GPa) were found to be closely related to the microstructure.

Computational insights into the relation between elements’ physical properties and mechanical properties of 3d, 4d, and 5d transition metal carbides via machine learning

Not much is known about the nature of hardness science in fundamental studies. In order to unravel the mystery on hardness science in fundamental studies, we have tried to gain computational insights into the relation between elements’ physical properties and mechanical properties of hard materials via machine learning. We have systematically analyzed two LASSO-regression machine learning models with regard to the Debye temperature and Vickers hardness values of 3d, 4d, and 5d transition metal carbides MC (M = Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt) in five different crystal structures (NaCl-type, WC-type, ZnS-type, CsCl-type, and NiAs-type structures), using first-principles calculations based on the density functional theory. Using machine learning, we have made it clear that the following explanatory variables are important to hardness: atomic number, Mendeleev number, Martynov–Batsanov electronegativity, heat of vaporization, thermal conductivity, equilibrium volume, and formation enthalpy. They can be divided into three categories: number of valence electrons, electronegativity, and thermal property, because atomic number and Mendeleev number belong to the role of number of valence electrons, and because heat of vaporization, thermal conductivity, equilibrium volume, and formation enthalpy fall into thermal property. Our results indicate that the number of valence electrons, electronegativity, and thermal property of materials are the factors in enhancing hardness.

Structure stabilization effect of vacancies and entropy in hexagonal WN.

The structural stability of hexagonal tungsten mononitride (WN) has been studied combining scanning transmission electron microscopy and first-principles calculations. The results show that the WC-type WN with vacancies of 6∼8 at% is more stable than the previously proposed MnP-type and NiAs-type structures. Due to the larger vibrational entropy of the WC-type WN, the vacancy concentration required to stabilize the WC-type structure is lower at high temperatures. The results demonstrate the importance of vacancies and configurational and vibrational entropies in the structural stability of compounds synthesized at high temperatures.

Enhanced Hardness in Transition-Metal Monocarbides via Optimal Occupancy of Bonding Orbitals.

An efficient strategy that can guide the synthesis of materials with superior mechanical properties is important for advanced material/device design. Here, we report a feasible way to enhance hardness in transition-metal monocarbides (TMCs) by optimally filling the bonding orbitals of valence electrons. We demonstrate that the intrinsic hardness of the NaCl- and WC-type TMCs maximizes at valence electron concentrations of about 9 and 10.25 electrons per cell, respectively; any deviation from such optimal values will reduce the hardness. Using the spark plasma sintering technique, a number of W1-xRexC (x = 0-0.5) have been successfully synthesized, and powder X-ray diffractions show that they adopt the hexagonal WC-type structure. Subsequent nanoindentation and Vickers hardness measurements corroborate that the newly developed W1-xRexC samples (x = 0.1-0.3) are much harder than their parent phase (i.e., WC), marking them as the hardest TMCs for practical applications. Furthermore, the hardness enhancement can be well rationalized by the balanced occupancy of bonding and antibonding states. Our findings not only elucidate the unique hardening mechanism in a large class of TMCs but also offer a guide for the design of other hard and superhard compounds such as borides and nitrides.

On the potential for superconductivity in ZrX (X = S and Te): a first-principles study

The phonon nature and superconductive properties of Zirconium chalcogenides (ZrS and ZrTe) in WC-type structure are evaluated leveraging density functional theory of first principles. In the WC-type structure, the phonon frequencies are positive in the Brillouin zone suggesting their stability in that region. The calculated zone center phonon dispersion curves and phonon density of states for ZrS and ZrTe are found to be in good agreement with other available theoretical results. Superconductivity in ZrS and ZrTe is explored in detail by measuring line widths of phonons, the spectral function of Eliashberg, electron-phonon coupling constant, and transition temperature. ZrS has a slightly higher Tc (3.71 K), compared to ZrTe with Tc (1.72 K).

Synthesis of Polycrystalline Diamond Compact with Selenium: Discovery of a New Se–C Compound**Supported by the National Natural Science Foundation of China (Grant No. 51402245), the Fundamental Research Funds for the Central Universities of China (Grant No. 2682016CX062), the China Scholarship Council (Grant No. 201707005071), the Shenzhen Peacock Plan (Grant No. KQTD2016053019134356), and the Guangdong Innovative & Entrepreneurial Research Team Program

Sintering of polycrystalline diamond with selenium was investigated under pressure of 6.5–10.5 GPa at a constant temperature of 1850 °C. A new carbon-selenium compound with a most plausible chemical formula of SeC and a WC-type hexagonal structure (space group ) has been discovered in the recovered samples sintered at 10.5 GPa and 1850 °C. Refined lattice parameters are as follows: a = 2.9277(4) Å, c = 2.8620(4) Å, V = 21.245(4) Å3. The diamond compacts hot-pressed at 10.5 GPa have excellent mechanical properties with a Vickers hardness of about 68 GPa at a loading force of 19.6 N. Diamond intergrowths observed in these samples may have benefited from the catalytic effects of Se/SeC on the nucleation and crystal growth of diamond.

Structural, elastic, and electronic properties of topological semimetal WC-type MX family by first-principles calculation**Project supported by the National Natural Science Foundation of China (Grant No. 51671193). All calculations have been performed on the high-performance computational cluster in the Shenyang National University Science and Technology Park.

Recently, the non-centrosymmetric WC-type materials (i.e., MoP, ZrTe, TaN, etc) have attracted extensive interest due to the discovery of their topological properties. By means of the first-principles calculations, here we have investigated the structural, thermodynamic, elastic, and electronic properties of the WC-type MX compounds (TiS, TiSe, TiTe, ZrS, ZrSe, ZrTe, HfS, HfSe, and HfTe). Among these nine compounds, five of them (TiS, ZrS, ZrSe0.9, ZrTe, and Hf0.92Se) have been experimentally synthesized to crystallize in the WC-type structure and other four members have never been reported. Our calculations demonstrated that they are all structurally, thermodynamically, and dynamically stable, indicating that all of them should be possibly synthesized. We have also derived their elastic constants of single crystalline and their bulk and shear moduli in terms of the R. Hill approximations. Furthermore, in similarity to ZrTe, all these compounds have been theoretically derived to be topological semimetals. Whereas TiS is unique because of the coexistence of the Dirac nodal lines (DNLs) and sixfold degenerate nodal points (sixfold DNPs), the other eight members are revealed to exhibit coexisted Weyl nodes (WPs) and triply degenerate nodal points (TDNPs). Their electronic and topological properties have been further discussed.

The rare earth metal hydride tellurides REHTe (RE=Y, La–Nd, Gd–Er)

Abstract The synthesis and crystal structure of a series of rare earth metal hydride tellurides with the composition REHTe (RE = Y, La–Nd, Gd–Er) is reported. These compounds have been obtained by the reaction of rare earth metal dihydrides (REH2) with elemental tellurium in sealed tantalum capsules at T = 700°C using cesium chloride (CsCl) as fluxing agent, which can be washed away with water due to the astonishing insensitivity of these hydride tellurides (REHTe) against hydrolysis. All of the compounds crystallize in the hexagonal space group P6̅m2 with a filled WC-type crystal structure, exhibiting a mutual trigonal-prismatic coordination of the heavy ions (RE 3+ and Te2−), while the hydride anions reside in the trigonal prismatic voids surrounded by three rare earth metal cations expanding their coordination pattern to a tricapped trigonal prism. This 1H-type crystal structure is compared with the 1H- and 2H-type structures of the respective hydride selenides (REHSe, RE = Y, La–Nd, Gd–Tm, Lu). Both hexagonal basic crystal structures can be derived from the AlB2-type structure as demonstrated in a Bärnighausen tree by group-subgroup relationships.

Geometries, electronic properties and stability of molybdenum and tungsten nitrides low-index surfaces

Motivated by the vital role played by transition metal nitride (TMN) composites in various industrial applications, the current study reports electronic properties, thermodynamic stability phase diagram, and vacancy formation energies of the plausible surfaces of NiAs and WC-type structures of δ3-MoN and δ-WN hexagonal phases, respectively. Low miller indices of various surface terminations of δ3-MoN and δ-WN namely, (100), (110), (111), and (001) have been considered. Initial cleaving of δ3-MoN bulk unit cell offers separate Mo and N terminations signified as δ3-MoN (100): Mo, δ3-MoN(100):N, δ3-MoN(111):Mo, δ3-MoN(111):Mo, and δ3-MoN(001):Mo. However, the (110) plane reveals mix-truncated with both molybdenum and nitrogen atoms i.e. δ3-MoN (110): MoN. Likewise, the δ-WN faces incur analogous surface terminations. Ab initio atomistic thermodynamic analyses predict that, N-terminated (111) and (100) slabs to be the most energetically favourable surface terminations amongst the explored surfaces of δ3-MoN and δ-WN, respectively. Evidenced by plotted density of states (DOS), bulk and surfaces of δ3-MoN and δ-WN display a metallic character. In terms of surface relaxation and reconstructions, most investigated surfaces experience mainly downward displacements of their topmost layers. Most notably, the relaxed Mo-termination in (111) and (100) surfaces of δ3-MoN demonstrate significant reconstructions resulted in the first layer to be solely truncated with nitrogen atoms instead of molybdenum in the un-relaxed geometry. Nevertheless, no surface reconstruction has been noticed in most of considered δ-WN configurations. Calculated Bader’s electronic charges reveal charge transfer from Mo/W atoms to N atoms, largely retaining the ionic bond nature in their bulk phases. Finally, vacancy formation energy (VFE) calculations showed that introducing nitrogen vacancies through the surface is an endothermic process. Furthermore, the energy required to create a vacant cite in the inner layers differ than that needed in the outer layers. Nitrogen-terminated slabs hold the highest concentrations. Results from this study should be useful when studying the activation of doubly and triply bonded molecules such as N2 at surface vacancies.

Hexagonal Phase Intergrown with the Tetragonal Weyl Semimetal TaAs

TaAs as a well-known topological Weyl semimetal has attracted intense interests in recent study. Our structural investigations on the microstructure properties of TaAs samples using Cs-corrected transmission electron microscopy and scanning transmission electron microscopy evidently show the presence of a hexagonal phase intergrown with the known tetragonal TaAs (NbAs-type). The structure of the hexagonal phase is identified to have the WC-type structure (space group P6m2) with hexagonal c-axis along the [100]tetra or [010]tetra direction. The crystallographic correlation between the hexagonal and tetragonal phase, as well as the relevant local atomic motions, has been carefully analyzed.

Structural, electronic and optical properties of hexagonal TaN compound

Structural and electronic properties of hexagonal Tantalum nitride (TaN) in CoSn and WC structures are studied using the first-principle calculations. Lattice constants and electronic band structures are in an excellent agreement with the available experimental and other theoretical values. TaN in both structures studied has a metallic nature and a strong hybridization of Ta 5d and N 2p are found from the spin density of states (DOS). Meanwhile, our LSDA+U calculations predicted a strong ferromagnetic state for CoSn-type structure and an obvious paramagnetic nature for WC-type structure. No phase transition are observed within cubic and hexagonal CoSn and WC structures under high pressures. Our results show WC-type TaN is the calculated ground-state structure among the three crystallographic structures studied under 120 GPa. Optical properties show that TaN in CoSn-type structure is a better dielectric material.

New crystal structure and physical properties of TcB from first-principles calculations**Project supported by the Science Foundation of Baoji University of Arts and Sciences of China (Grant No. ZK11061) and the Natural Science Foundation of the Education Committee of Shaanxi Province, China (Grant Nos. 2013JK0637, 2013JK0638, and 2014JK1044).

By combining first-principles calculations with the particle swarm optimization algorithm, we predicted a hexagonal structure for TcB, which is energetically more favorable than the previously reported WC-type and Cmcm structures. The new phase is mechanically and dynamically stable, as confirmed by its phonon and elastic constants calculations. The calculated mechanical properties show that it is an ultra-incompressible and hard material. Meanwhile, the elastic anisotropy is investigated by the shear anisotropic factors and ratio of the directional bulk modulus. Density of states analysis reveals that the strong covalent bonding between Tc and B atoms plays a leading role in forming a hard material. Additionally, the compressibility, bulk modulus, Debye temperature, Grüneisen parameter, specific heat, and thermal expansion coefficient of TcB are also successfully obtained by using the quasi-harmonic Debye model.

Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction

AbstractMolybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β‐Mo2C with an Fe2N structure. Here, four phases of Mo‐C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine–metal oxide composite material including γ‐MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X‐ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ‐MoC. γ‐MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.

Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction.

Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β-Mo2C with an Fe2N structure. Here, four phases of Mo-C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine-metal oxide composite material including γ-MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X-ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ-MoC. γ-MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.

Phase transition, elastic and thermodynamical properties of TcC under high pressure from first-principles calculations

Phase transition, elastic and thermodynamical properties of technetium monocarbide (TcC) in the WC, NiAs, NaCl, CsCl, and ZnS structures have been studied by full-potential linearized augmented-plane-wave (FP-LAPW) theory. From the enthalpy difference between the five structures, we predict that the structural phase transition from WC-type structure to CsCl-type structure occurs at ca. 411.4 GPa. Meanwhile, it is found that the NiAs, NaCl, and ZnS types are not stable phases in whole pressure ranges considered. In particular, for the first time we calculate the elastic properties and phase transition of TcC under high pressures. For WC TcC and CsCl TcC structures, the elastic constants, bulk modulus, shear modulus, and Young's modulus are predicted to increase monotonically with increasing pressure. The Vickers hardness of TcC under high pressure is estimated. The brittle–ductile behavior of TcC has been obtained by Pugh's criteria. Finally, the Debye temperature, isochoric heat capacity and thermal expansion coefficient are predicted by the quasiharmonic Debye model method.

High-pressure synthesis and compressive behavior of tantalum nitrides

WC- and NaCl-type tantalum mononitrides and hexagonal Ta5N6 were prepared at high pressure and temperature, and their compressive behaviors were examined using in situ high-pressure X-ray diffraction. Comparison of the formula volumes of the tantalum mononitrides indicated that the NaCl type was the densest phase. The P–V data showed that the WC-type structure had the highest bulk modulus value (K0 = 351(1) GPa). An analysis of the compression properties in terms of the crystallographic characteristics of the structures indicated that a prismatic polyhedral array with face-sharing connectivity was responsible for the incompressible nature of these tantalum nitrides.

First-principle calculations of high-pressure phase transformations in RuC

The structural stabilities of RuC under high pressure have been studied by using first-principle calculations. Results show that RuC transforms from the ZB-type (zinc-blende-type) structure to a tetrahedral structure with space group I4mm at . The I4mm structure constructed by RuC5 pyramids is stable up to , above which the WC-type structure becomes energetically more favorable. An increase of the coordination number from 4 to 5 and then to 6 is observed accompanied by the phase sequence of ZB type WC type. Band structure calculations reveal that the ZB-type phase is a semiconductor while the I4mm and WC-type phases are metallic. Moreover, the mechanical properties of RuC in all three phases are discussed.

Phase stability and elastic properties of (W0.5Al0.5)C phase with a novel NiAs-type structure

Using density-functional theory, we show that the NiAs-type is a more favorable structure for the (W0.5Al0.5)C phase than the experimentally proposed WC-type structure when we compare the thermodynamic, dynamic and elastic properties of the two types.

Phase stability and physical properties of technetium borides: A first-principles study

By first-principles calculations, we establish the phase stability of technetium borides with various stoichiometries. Moreover, a new phase of TcB has been predicted using a newly developed particle swarm optimization (PSO) algorithm, which belongs to the orthorhombic Cmcm space group. The new phase is both mechanically and dynamically stable, as verified by the calculations of its elastic constant and phonon dispersion. The formation enthalpy–pressure diagrams reveal that the predicted TcB is more energetically favorable than the previously proposed WC-type structure in a pressure range from 0 to 100GPa. Our calculations on the enthalpy–pressure relationship and convex hulls have demonstrated that the experimentally observed Tc3B, Tc7B3, and TcB2 are stable against decomposition into other components both at zero and high pressure, and the predicted TcB becomes the most stable phase above 8GPa. An analysis of the elastic properties and Debye temperature shows that TcB2 is a potential hard material. The electronic localization function and electronic structure provide that the strong Tc–B and B–B covalent bonds are the main reason of its high hardness.

New compounds and phase equilibria in the Zr–Ti–P system

The interaction between components in the Zr–Ti–P system in the range of up to 67at.% P was analyzed and the isothermal section diagram at 1070K was constructed from X-ray structural and EDS analysis. Solid substitutional solutions based on the binary phosphides M3P with the Ti3P-type structure (Ti3.0−2.3Zr0−0.7P and Zr3.0−2.5Ti0−0.5P) were found. The phosphide (Ti,Zr)2P with the Co2Si-type structure is homogeneous at the Zr1.2−0.6Ti0.8−1.4P composition. From powder X-ray diffraction methods, the crystal structure of ZrTiP0.68 was shown to be of the Zn3Cu-type (space group P-6, a=0.47600(1), c=0.30038(1)nm, RI=0.0444); HfTiP0.68 is isostructural. These compounds are homogenous in the range Zr1.0−1.5Ti1.0−0.5P0.68 and Hf0.9−1.2Ti1.1−0.8P0.68. From single-crystal methods, the structure of Zr0.93Ti0.07P0.93 was determined to belong to the WC-type structure (space group P-6m2, a=0.3004(2), c=0.3117(3) nm, RF=0.0228).