Universal ground state hexagonal phases and mechanical properties of stoichiometric transition metal tetraborides: TMB4 (TM = W, Tc, and Re)

Using particle swarm optimization technique on crystal structural prediction, a universal hexagonal MoB4-type structure was uncovered for WB4, TcB4, and ReB4. This MoB4-type structure is energetically much superior to the previously proposed WB4-type structure and stable against decomposition into the mixture of TM and B at ambient conditions. The Young’s modulus and shear modulus as a function of crystal orientation for the TMB4 (TM = W, Tc, and Re) have thus been systematically investigated. TMB4 within this hexagonal structure are found to be ultra-incompressible and hard due to their high bulk modulus and large hardness. Especially, they exhibit an unusual incompressibility along the c axis, close to that of diamond. Their ultra-incompressibility is attributed to a staking of B–TM–B “sandwiches” layers linked by strong covalent TM–B bonding, which is manifested by the PDOS and ELF analysis.