Structural determination and physical properties of 4d transitional metal diborides by first-principles calculations

The structural determination, thermodynamic, mechanical, dynamic and electronic properties of 4d transitional metal diborides MB₂ (M = Y-Ag) are systematically investigated by first-principles within the density functional theory (DFT). For each diboride, five structures are considered, i.e. AlB₂-, ReB₂-, OsB₂-, MoB₂- and WB₂-type structures. The calculated lattice parameters are in good agreement with the previously theoretical and experimental studies. The formation enthalpy increases from YB₂ to AgB₂ in AlB₂-type structure (similar to MoB₂- and WB₂-type). While the formation enthalpy decreases from YB₂ to MoB₂, reached minimum value to TcB₂, and then increases gradually in ReB₂-type structure (similar to OsB₂-type), which is consistent with the results of the calculated density of states. The structural stability of these materials relates mainly on electronegative of metals, boron structure and bond characters. Among the considered structures, TcB₂-ReB₂ (TcB₂-ReB₂ represents TcB₂ in ReB₂-type structure, the same hereinafter) has the largest shear modulus (248 GPa), and is the hardest compound. The number of electrons transferred from metals to boron atoms and the calculated densities of states (DOS) indicate that each diboride is a complex mixture of metallic, ionic and covalent characteristics. Trends are discussed.