First-principles Study Of Typical Superhard And Multifunctio Nal Transition Metal Borides

Superhard materials have attracted considerable attention in bothfundamental and technological applications due to their superiormechanical properties such as high melting temperature, high hardness,high electrical and thermal conductivity. Previously, it was generallyaccepted that the superhard materials are those strongly covalent bondedcompounds formed by light elements (B, C, N, and O), such as diamond,c-BN, BC5, and BC2N etc. These superhard materials are easily to formstrong three-dimensional covalent bonding networks. Although diamondis the known hardest material with a measured hardness at60120GPa,but it reacts easily with iron-based materials. The hardness of cubic boronnitride (c-BN) is second only to that of diamond. However, it can besynthesised only under high pressure and high temperature conditionswhich needs great cost. Therefore, great efforts have been devoted toexploring novel hard and ultra-incompressible materials. Recently, it was reported that partially covalent heavy transition metal(TM) boride, carbide, nitride, and oxide are found to be good candidatesfor superhard materials, such as ReB2, OsB2, CrB4and WB4. Thesereports revealed that they all possess high bulk and shear moduli. Becausethe compounds formed by transition metal and light elements usuallypossess high valence electron density and directional covalent bonds, andthose covalent bonds are strong enough to improve the mechanicalproperties. Moreover, d valence electrons are considered to contribute tothe hardness of transition-metal compounds. Further, these materials canbe synthesized under lower pressure which leads to the low-cost synthesiscondition and this is beneficial to their applications. Therefore, thesepioneering studies open up a novel route for the search of novel superhardmultifunctional materials.Very recently, two transition metal tetraborides MnB4and CrB4havebeen synthesized. Theoretical results show that boron-rich CrB4andMnB4exhibit superior rigidity with Vickers hardness of43and40GPa,respectively. So the other5d transition metal borides with higher boronconcentration can also be expected to have good mechanical properties.The bulk modulus of transition metal tantalum is very close to that oftransition metal chromium. So it can be expected that the boron-richtantalum borides may be a potential superhard material. Experimentally,the crystal structures of tantalum borides (TaB, Ta2B, TaB2, Ta3B4) wereconfirmed at ambient pressure. Experiments reported that TaB2has a high melting point (3200℃), high hardness (24.5GPa), and good thermal andelectrical conductivity. Theoretical studies revealed that the TaB withCmcm symmetry is mechanically stable. The estimated Vickers hardnessof TaB is28.6GPa. These reports indicate that tantalum borides areultra-incompressible and hard materials. However, there is lack of reportson systemic studies of boron-rich tantalum borides. Manyfundamental aspects of boron-rich tantalum borides are still not wellunderstood because of its complex chemical behavior. Therefore, adetailed study of boron-rich tantalum borides is undoubtedly necessaryand may bring new insight for further understanding its uniquemechanical properties. In this study, we explore the structural stabilities,elastic properties, electronic structure of TaB4by using ab initiocalculations based on density functional theory. A monoclinic C2/mstructure TaB4is uncovered, which is energetically much more preferablethan the structures of recently proposed transition metal tetraborides. Wename it as m-TaB4. The lattice parameters, total energy, bulk modulusand hardness were also been investigated for this novel material. Them-TaB4exhibits higher bulk modulus259GPa, shear modulus220GPaand high hardness (29GPa) among transition borides.Over the past decades, ReB2, OsB2and RuB2have been synthesizedand identified as superhard materials, and theoretical studies support theresults. ReB2draws special interest. OsB2and ReB2form different latticestructures: OsB2has an orthorhombic RuB2-type structure (space group Pmmn), and ReB2crystallizes with the hexagonal ReB2-type structure(space group P63/mmc). The hardness of ReB2is48GPa at a load of0.49N. Could the other5d transition metal borides also present such a goodnature? Ir and Re are neighboring5d transition metals. And theexperimentalists synthesized a pure phase IrB1.35in X-ray diffractionexperiment. The Vickers hardness of IrB1.35was18.249.8GPa,depending on the loads ranging from0.49to9.81N. Besides, we alsofind superhard IrB1.1film, which has a high Vickers hardness43GPa.Recently, Wang et al. reported that IrB2in the orthorhombic phase withPmmn space group was most stable phase at ambient pressure. Accordingto this study, the bulk modulus and shear modulus of IrB2are277GPaand108GPa. The IrB2could be a good superhard material. Therefore, thestudy of high-pressure structures and phase transitions of IrB2may bringfurther understanding of TM compounds and their behavior. In this study,we explore the structural stabilities, elastic properties, electronic structureof IrB2by using ab initio calculations based on density functional theory.A monoclinic C2/m structure IrB2is uncovered, which is energeticallymuch more preferable than the structures of recently proposedPmmn-type. We name it as m-IrB2. The new phase is dynamically andmechanically stable. Phase stability, electronic structures, elasticproperties of different structures are compared and analyzed in detail. Theboron layers’s quadrilateral networks form a strong covalent bondingfeature, and strong covalent bonds suggest m-IrB2is a low compressible and hard materialThe borides of rhodium are well known for their high meltingtemperature and hardness. The measured Vickers hardness of bulk RhB1.1was7-22.6GPa, when the loads ranging from0.49to9.81N. Later, the1.0μm thick RhB1.1film was studied by X-ray diffraction and it possessesa hardness of44GPa. Past studies have identified two stoichiometriccompositions: RhB (hexagonal structure, No.194, P63/mmc). Wang et al.reported that when the pressure exceeds22GPa, RhB transforms fromhexagonal RhB (anti-NiAs type) to the orthorhombic Pnma space group(FeB type), Rh2B (No.62, Pnma) has been determined that it possesses anorthorhombic structure. In1953, Richard et al. studied the crystalstructure of Rh2B from X-ray rotation and Weissenberg photographs.Over the past60years, experimental equipment and technology havebeen improved dramatically, but there is no report about Rh2B in theseyears. This led us to the idea that if phase transition may occur in theRh2B, which may bring about novel properties. Detailed structural,mechanical, and electronic properties theoretical investigations of Rh2Bare also seldom. Are there compounds with higher boron contents? Highboron compounds did not reveal any new phases so far. The results areworth making the effort. In this article, we report two new phases forRh2B and RhB2by the first-principles calculations. Our results show thatthe predicted new phase of Rh2B belongs to the monoclinic P21/m phase,which is energetically much more stable than the previously proposed Pnma structure in experiments. At the pressure of about39GPa, theP21/m phase transforms to C2/m phases. While the structure type of thenew phase of RhB2also belong to the monoclinic P21/m phase. Both ofthe two phases are dynamically and mechanically stable at ambientpressure. Further calculations are performed to study the properties ofthose high-pressure phases.