Mechanical Properties Of Os_mb_n And Ru_mb_n Compounds: A First-principles Study

Superhard materials have a large number of industrial applications for abrasives andcutting tools, and wear-resistant and protective coatings. Although diamond is still thehardest material until now, its intrinsic weakness is also distinct. Diamond cannot cutthrough steel or ioron due to its thermal instability, which limits its applications in theindustry. Other covalently bonded compounds only made up of B, C, N, and O have highhardness comparable of diamond, but their production costs are too high owing to themore rigorous synthesis condition （higher temperature or pressure） in comparison withdiamond.Currently, synthesizing some strong covalently bonded compounds mainlycomposed of transition metal （i.e., W, Os, and Ir） and B, C, N, and O has become a newpathway to design new superhard materials with hardness higher than40GPa. In Os-Bsystem, OsB₂, as a potential superhard material, has been extensively investigated, butrecent experimental and theoretical studies have indicated that OsB₂cannot be utilized ina superhard material due to its significant anisotropy in hardness. The Os-B compoundswith higher B content are expected to yield higher hardness due to the exisitence ofstrong B-B covalent bonds. However, structural information of the Os-B compounds withhigher B content is still very scarce and the phase diagram of Os-B system still need tobe improved. Whether the Os-B compounds with higher B content exist is still an openquestion.In the present work, we have performed a systematic structure research for Os_mB_ncompounds using the ab intio evolutionary structure prediction based on the geneticalgorithm. Two previously unreported stable structrure with space group R3m and P-6m2for Os₂B₅and OsB₃are first predicted. Os₂B₅and OsB₃have the negative heat offormation, which indicates that they can probobaly be chemically synthesized by pure Osand B. Elastic constants cijand other mechanical parameters including bulk modulus B,shear moudulus G, Young’s modulus E, Possion’s ration ν of Os-B comounds （includingOsB, Os₂B₃, OsB₂, Os₂B₅, and OsB₃） have been further calculated using thefirst-principles total-energy calculations and by applying small strain on different thelattice cells of Os-B system. Our results are in good agreement with the availableexperimental and theoretical values. Further calculations on hardness H havedemonstrated that the calculated values of H for Os-B compounds increase withincreasing B content. Os₂B₅and OsB₃with high hardness of34.4and36.9GPa canalmost be considered as new potential superhard materials. Detailed analysis of electronic structure have shown that hardness enhancement in the Os-B compounds withhigher B content （i.e., Os₂B₅, OsB₃） is mainly attributed to the occurrence of strong B-Bcovalent bonds and the supersession of ductile Os-Os metallic bonds by partiallycovalent Os-B bonds along some specific axial direcions. While in the Os-B compoundswith lower B content （i.e., OsB, Os₂B₃）, the existence of much more Os-Os bonds is notgood for enhancing their harndess, although they have higher bulk modulus and shearmodulus in comparison with Os₂B₅and OsB₃, which leads to yield the mechanicalbehavior similar to the pure Os.Considering the structural similiarity between Ru-B and Os-B systems, we havefurther investigated the mechanical properties of Ru_mB_nusing the same structure modelsand computational methods. The calculated results show that the values of cij, B, G, E,and ν of Ru_mB_nare lower than those of Os_mB_nwith the same crystal structures. However,the variation of these values in Ru-B system with increasing B content is different fromthat in Os-B system. In Ru-B system, Ru2B₅with the lowest values of B/G and ν exhibitssignificant brittleness; while RuB₃with the highest B content has the largest values ofB/G and ν, which is entirely different from the isostructural OsB₃. The significantdifference in mechanical properties between Ru_mB_nand Os_mB_ncan partially be attributedto the difference in chemical bonding between4d-Ru-B and5d-Os-B. The deepermechanism, however, needs to be further investigated in the future work.