Orthorhombic ScB3 and hexagonal ScB6 with high hardness

Transition metal borides (TMBs) have received great interest since these intriguing borides could be considered potential applications for designing exotic materials with distinguished properties such as superhardness. The search for TMBs regarding high hardness value remains urgent. Given that Sc is the lightest transition metal element that often exhibits unique properties, in this paper, we undertake a comprehensive investigation of the Sc-B system at high pressures using the state-of-the-art structure search method within the framework of first-principles electronic structure. As a result, our structure searches identified several stable Sc-B phases (i.e., ScB, ScB3, ScB4, and ScB5) as well as a metastable ScB6. Strikingly, among these scandium borides, Pnma ScB3 contains a boron framework with open channels, while ScB6 possesses interlinking boron trimer units. Remarkably, our simulations suggest the hard features of Pnma ScB3 and P6(3)mc ScB6 with high simulated Vickers hardness values of 38.3 and 39.8 GPa, respectively, which are mainly attributed to their peculiar strong covalent boron network. The excellent stability and high hardness of ScB3 and ScB6 render them promising candidates for superhard materials. Additionally, an orthorhombic boron allotrope o-B-12 is predicted by removing Sc from Pnma ScB3, where this structure exhibits an estimated hardness value of 24.9 GPa and superconducting critical temperature T-c of 2.9 K at 1 atm. In this paper, we advance the existing knowledge of TMBs as well as provide implications for the search for TMBs with unique properties.

Automated summary

This paper presents a comprehensive investigation of the Sc-B system at high pressures using first-principles electronic structure calculations. Stable and metastable scandium borides were identified, with one boride having an open channel boron framework and another having interlinking boron trimer units. Simulations showed that these compounds exhibit high hardness values, indicating their potential as superhard materials.