Vacancies and substitutional defects in multicomponent diboride Ti_0.25Zr_0.25Hf_0.25Ta_0.25B₂: first-principle study

We study the hard and electrically conductive multicomponent diboride Ti_0.25Zr_0.25Hf_0.25Ta_0.25B₂with high thermal stability byab-initiocalculations. We focus on the effect of defects (either vacancies or C atoms, both relevant for numerous experiments including our own) on material characteristics. Different types, concentrations and distributions of defects were investigated, and the configurations leading to the lowest formation energies were identified. We show that the replacement of B by C is more unfavorable than the formation of B vacancies. We show that vacancies prefer to coalesce into a larger planar void, minimizing the number of broken B-B bonds and the volume per atom, while carbon substitutions at boron sites do not prefer coalescence and tend to minimize the number of C-C bonds. We show the effect of vacancies on mechanical and electronic properties, and use the results to explain experimental data.