Ordered structure and mechanical properties of ternary Sc0.5TM0.5B2 (TM = Ti,V, Zr) alloys under high pressure

The structural and mechanical properties of ScB₂ and Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are investigated in the pressure range of 0–150 GPa based on density functional theory. The ground state structures of ScB₂ are screened out by structural substitution, and the P6/mmm is determined as the initial structure of alloying research according to structural stability. The structures of alloy generation and recognition (SAGAR) code combined with first-principles calculations selected the stable structures of Sc_0.5TM_0.5B₂ (TM = Ti, V) and Sc_0.5Zr_0.5B₂ alloys as ordered structure types Ⅰ and Ⅱ, respectively. The formation enthalpy, phonon dispersion and elastic constants demonstrate that Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are thermodynamically, dynamically and mechanically stable. In the whole pressure range, the elastic moduli of Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) increased significantly compared with ScB₂. This indicates that the introduction of TM improves the mechanical properties of ScB₂. The Vickers hardness (H_V) of the ScB₂ ground state is 42.4 GPa, and the H_V of the Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are increased to 47.8, 50.3 and 44.8 GPa, respectively. The electronic structures and chemical bonding reveal that the Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys have stronger B–B, B–TM covalent bonds, charge interaction, and higher valence electron density, which significantly improves the hardness. The results show that ScB₂ and Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are potential superhard multifunctional materials.