Using first principles plane-waves pseudopotential method, the thermodynamic stability, the
elastic constants, and structural phase transition of technetium diboride (TcB2) under pressure are revealed,
as well as the role of metallic bond on its hardness. Seven possible structures are chosen to
probe, including marcasite-type orthorhombic, OsB2-type orthorhombic, simple tetragonal (ST-type),
MoB2-type hexagonal, AlB2-type hexagonal, ReB2-type hexagonal, and diamond-type cubic lattices.
The calculations demonstrate that the ReB2-TcB2 phase holds the most energetically stable structure in
a larger range of pressure. According to the criteria of the lowest Gibbs energy, the phase transition point Pt = 147.5 GPa
between the ReB2-TcB2 and AlB2-TcB2 phases is firstly determined. Furthermore, Mulliken overlap population analysis
allows us to use a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bond.
The superior performance and large hardness (34.0 GPa) of ReB2-TcB2 suggest that it is an incompressible, anisotropic,
brittle and hard material.