Phase diagram exploration of Tc–Al–B: from bulk Tc2AlB2 to two-dimensional Tc2B2

Abstract

In this study, the ternary phase diagram of the Tc–Al–B system is constructed by a combination of an evolutionary algorithm and density functional theory calculations. Four novel phases are predicted, including three binary compounds (P Al₇B₁₅, Cmcm TcAl₂, and C2 TcAl₃) and one ternary compound (Cmmm Tc₂AlB₂). All predicted structures are mechanically, dynamically, and thermodynamically stable. Among the predicted phases, P Al₇B₁₅ resembles the experimental structure of Al_0.93B₂ and Cmmm Tc₂AlB₂ corresponds to the 212-type MAB phase. Due to the in- and out-of-plane anisotropic chemical bonding in Cmmm Tc₂AlB₂, a tetragonal two-dimensional (2D) Tc₂B₂ structure could be possibly exfoliated by chemical removal of Al atoms. The electronic structure calculations indicate that the 2D Tc₂B₂ structure and its potential layered precursors are all metallic. Furthermore, the chemical reactivity of H, F, O and, OH ligands with the 2D Tc₂B₂ surface is studied and the associated 2D surface-functionalized Tc₂B₂ derivatives are found to be metallic. It is revealed that the F and O functional groups strengthen the surface atomic layer of 2D Tc₂B₂ and enhance the Young's moduli.