Structures And Physical Properties Of Novel Transition Metal Borides

The synthesis of traditional superhard materials(such as,diamond and cubic boron nitride)is mostly sintered by high temperature(>2000?)and high pressure(>7.7GPa).Several shortcomings also restrict their applications,for example,diamond cannot be used to cut ferrous materials and cubic boron nitride is virtually impossible to shape by cutting or polishing.Transition metal light element compounds,especially transition metal borides,with high valence electron density and strong covalent bond networks,have received extensive attention as potential substitutes for traditional superhard materials.Rhenium diboride,as a typical transition metal boride,has further stimulated research interest because of its Vickers hardness of more than 40 GPa(?48 GPa)under small load and asymptotic load-independent hardness close to 30 GPa.However,osmium,as a platinum group metal,is expensive,resulting in high synthesis cost.Through the synthesis of a series of transition metal borides,normal tungsten tetraboride(hereafter called tungsten tetraboride)with price advantage and excellent mechanical properties is favored.Experiments results show that the Vickers hardness of tungsten tetraboride is comparable to or even better than that of rhenium diboride.The confirmation of crystal structure is helpful to fundamentally understand the mechanical properties of materials.However,the long-term unknown and objection of crystal structure of tungsten tetraboride hinder the renewal of the understanding of the mechanical mechanism and design ideas of this series of compounds.Currently,the asymptotic load-independent hardness of binary transition metal borides(such as,WB₄ and ReB₂)still can not reach the superhard material threshold(<40 GPa).Hence,the researchers further broadened the research scope of transition metal borides,and synthesized many ternary transition metal borides or solid solutions by doping means based on a series of binary transition metal borides.Exploring new superhard materials in ternary transition metal borides is an interesting and broad research direction,which is worthy of further exploration.Based on first-principles calculations,this paper systematically studies the crystal structure and physical properties of transition metal borides,and obtains the following innovative results:1.Recently,advanced experimental measurements[Andrew T.Lech,et al.,PNAS,112(11),3223-3228(2015)]combining X-ray and neutron diffraction techniques provided strong evidence for the existence of boron trimers(triangular B3 unit)occupying partial W vacant sites,and proposed that the resulting compound has the stoichiometry of“WB_4.2”.Based on this,we have established a viable structure for the long-term pending highest boride of tungsten WB_4.2 by a tailored crystal structure search approach utilizing structural constraints based on our group developed CALYPSO software.This structure is energetically,dynamically,and thermodynamically(meta)stable,producing XRD and neutron spectra in excellent agreement with measured data.Our stress-strain calculations show that the B15 unit composed of a boron trimer and adjacent layer of two boron six-membered ring can effectively strengthen the bonding and generate higher stress to resist diverse deformations.2.Considering the similar atomic radii of W(1.37(?))and Mo(1.36(?)),chemical bonding patterns,and phase diagram,it is reasonable to adopt the similar structural assignment containing boron trimers in the chemical stoichiometry of 1:4.2 for molybdenum borides.Fisrt,candidate structures of MoB_4.2 were theoretically investigated using tailored crystal structure search approach utilizing structural constraints as implemented in the CALYPSO code.The most thermodynamically stable MoB_4.2 structure is identical to that of the highest boron content tungsten borides and well reproduces the experimental XRD patterns.We explored the related crystal structures of W_1-xMo_xB_4.2 based on the Mo-doped tungsten tetraborides experiments and found a series of thermodynamically stable W_1-xMo_xB_4.2crystal structures.It is noted that the Mo atoms tend to locate close to boron trimers,while the reverse applies to W atoms in ternary W_1-xMo_xB_4.2.This phenomenon stems from the signifificant difference in the electropositivity of W and Mo atoms.The reported structural and mechanical calculations demonstrate that the basic physical properties of these low-energy structures W_1-xMo_xB_4.2 largely follow the Vegard's law.3.Previous experimental results show that some ternary borides or solid solutions by rationally introducing other transition metals into binary transition-metal borides can effffectively improve the strength and hardness of matrix materials.Considering the high hardness and excellent oxidation resistance,TaB₂ was chosen as the matrix material for introducing other transition metals.Here,Sc is selected as substitute element with the consideration of the prominent difffferent in atomic radii and valence electrons between Sc(1.61(?),3p⁶4s²3d³)and Ta(1.43(?),5p⁶6s²5d³)atoms,and thus a set of ternary transition-metal borides of Sc_xTa_1-xB₂,(x=0.0?1.0)are designed via a multicomponent strategy.The relative formation energies of Sc_xTa_1-xB₂ are found to be negative in all proportions and the thermodynamically stable structures of Sc_0.5Ta_0.5B₂with equiatomic proportions for metal reaches maximum negative value.Our systematic assessment show that Sc_0.5Ta_0.5B₂ has the most excellent mechanical properties.It is noted that the ideal pure shear strength of the Sc_0.5Ta_0.5B₂ is above 40GPa,which indicates that it will be a potential superhard material.The density of states shows that the pseudogap of Sc_0.5Ta_0.5B₂ almost exactly fall on the Fermi level,indicating the improvement of the material stability.The chemical bonding analysis indicate that the intrinsic strengthening effect of Sc_0.5Ta_0.5B₂ probably stems from the strengthening of B-B bond.