Theoretical study on mechanical and electronic properties of ternary diborides Sc0.5V0.5B2, Sc0.5Nb0.5B2 and Sc0.5Ta0.5B2

Abstract

Owing to the high thermal stability and superior mechanical properties, the transition-metal (TM) diborides have been proved to be a class of promising hard materials used as protective films. In the present work, a new type of ternary diborides Sc0.5V0.5B2, Sc0.5Nb0.5B2 and Sc0.5Ta0.5B2 alloyed by doping the TM scandium into VB2, NbB2 and TaB2 are systematically investigated using first-principle density functional theory (DFT) calculations. These ternary systems all possess higher hardness of up to about 45 GPa relative to their parent binary diborides ScB2, VB2, NbB2 and TaB2, suggesting they could be viewed as promising candidates of superhard materials. An improved structure searching approach was employed to explore the potential stable and metastable structures of them under the pressure of 0 ∼ 100 GPa. The hexagonal AlB2-type structure (P6/mmm) was confirmed to be their ambient phase, which can retain the stability up to 100 GPa. Meanwhile, their ideal tensile and pure shear strengths all reached about 40 GPa, demonstrating their superhard nature. The origin of the enhanced mechanical properties of these ternary systems was resolved in details by electronic analysis at fundamental level. Thanks to the introduction of the Sc, the p-d interactions between TM and B atoms are greatly enhanced. This remarkable enhancement of p-d coupling hybridization together with the pure covalent B-B bonds results in their high hardness property.