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

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 (P1̄ 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, P1̄ Al7B15 resembles the experimental structure of Al0.93B2 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.