Electronic Structure And Hardness Of The Transition Metal Osmium And Its Compounds: First-principles Calculations

Superhard materials are useful in a variety of industrial applications, such as abrasives, cutting tools, and coatings due to their superior properties of higher compressional strength, thermal conductivity, refractive index, and chemical stability beside higher hardness. Hence, the researches on superhard materials are always the hot issues. Diamond and cubic boron nitride (c-BN) are the usual superhard materials to our knownledge. However, diamond has a major drawback in that it reacts with iron and cannot be used for machining steel; c-BN does not react with iron and can be used for machining steel. But it does not exist in nature and is prepared under high-pressure and high-temperature conditions. Its synthesis is more difficult, and it is impossible to prepare large crystals. Therefore, the industry is looking for new superhard materials that will need to be much harder than present ceramics (Si3N4, Al2O3, TiC).In general, two groups of materials are powerful candidates for superhard materials: (1) the strongly covalent-bonded solids, such as diamond, cubic boron nitride (c-BN), carbon nitrides, and carbon dioxide. (2) the transition metals and their borides, carbides, nitrides, and oxides. Recent years, researchers found that transition metallic osmium has a very high bulk modulus comparable to diamond. This discovery has attracted people's broad attention. The hardness of metallic osmium is low, but it has been found that it can be converted into hard materials by combining with small covalent bond-forming atoms such as boron, carbon, oxygen, or nitrogen. Therefore, the osmium borides, carbides, nitrides, and oxides have become the most focus materials in the field of superhard materials.In this work, we carry out density functional calculations on the electronic properties and theoretical hardness of metallic osmium and its borides, carbides, nitrides, and oxides, using the highly accurate full potential linearized augmented plane wave method as implemented in the Wien2k code, and using"Atoms in molecules"theory for electronic properties; using"strain-stress theory"for elastic behavior; using"band filling theory"for phase stability. We have explained the reason for so high bulk modulus of transition metallic osmium reasonably, analyzed and discussed the physical properties of osmium compounds, and calculated their theoretical hardness. In this research, we have obtained some innovative results as follows.(1)We found that the high bulk modulus of transition metal osmium results from its high cohesive energy and charge density at bond critical points (BCPs). We investigated the atoms near osmium in elements periodic table systematically, which contain 4d and 5d electrons, and found that the values of cohesive energy and charge density between atoms are the highest for metallic osmium in all of these metals. This result demonstrated that the cohesive energy and charge density at BCPs play an important role in determining high bulk modulus of materials.(2) According to the key criteria for mechanical stability of a crystal, it can be seen that the OsC and OsN in NaCl structures are unstable under the ambient condition. For the cubic crystals and hexagonal crystals, we can estimate their mechanical stability by elastic constants. The OsC and OsN in NaCl structures are unstable under the ambient condition due to c 44 <0.(3) According to band filling theory, the relative phase stability of osmium compounds in different structures were confirmed by analyzing their electronic structures. And it can be seen that the bonds in osmium compounds are of the unusual mixtures of metallic, covalent, and ionic from bonding behavior. From the DOS plots, first of all, they all indicate metallic behavior because of finite N(EF) at Fermi level; Second, it can be seen that the electronic structure of these compounds are governed by a strong hybridization between the Os-d and B (C or N)-p states, while with a rather small contribution from the Os-p and B (C or N)-s states. This strong hybridization also indicates the existence of strong covalent bonding. Third, all the DOS plots show that there is a deep valley called pseudogap near the Fermi level, which actually separate bonding states from nonbonding (or antibonding) ones. According to the band filling theory, filling bonding or antibonding states will increase or reduce the cohesion (or stability). Similarly, the phase stability will depend upon the band filling of the bonding states. We denote the width of the occupied states by Wocc and the width of the bonding states, Wb, as the distance from the bottom of the band to EF and to the pseudogap, respectively. Hence, Wocc / Wb can be adopted to evaluate the occupied portion of the bonding states, i.e., the banding filling of the bonding states. The phase is relative stable when the Wocc / Wb value is close to 1. The results demonstrate that the stable phase for OsB, OsC, and OsN are WC structure.(4) In this work, for the first time to our knowledge, two different ab initio method for the calculation of the hardness were implied for osmium compounds. The two results are agreement with each other, and we found that there is no direct relationship between hardness and bulk modulus. Hardness is proportional to bond strength, i.e. the more bond strength is, the more hardness is. In the view of the hardness of osmium compounds, they are not superhard materials (hardness >40GPa). The research on hardness of osmium compounds have an important signification in exploring new kinds of superhard materials in experiment, and establish the basis for further research in theoretically.