First-principles analysis of improved thermodynamic stability and mechanical properties in pseudo-binary Y1−xVxB2 alloys

This work explores a possibility of improving the mechanical behavior and thermodynamic stability of AlB₂ -type YB₂ through alloying with isostructural VB₂ using first-principles calculations. The analysis derived from the cluster-expansion model suggests Y_0.5V_0.5B₂, whose atomic configuration is represented by periodically alternating YB₂/VB₂ layers in the ¡0001¿ direction, is the only solution in the pseudo-binary YB₂–VB₂ system predicted to be stable from the thermodynamic viewpoint. By evaluating the influence of lattice dynamics on the Gibbs free energies of superlattice-structured Y_0.5V_0.5B₂ and its constituent compounds within the quasiharmonic approximation, the thermodynamic stability, elastic properties, and hardness at a given pressure and temperature of the three diborides can be accessed. The results reveal, at a given temperature, isotropic compression of the diborides to high pressures enhances the stability of Y_0.5V_0.5B₂ measured relative to YB₂ and VB₂, while raising the temperature at a given applied pressure can increasingly result in a driving force toward separation of Y_0.5V_0.5B₂ into YB₂ and VB₂. The thermodynamic stabilization of superlattice-structured Y_0.5V_0.5B₂, despite large distinctions in atomic radius and electronegativity between Y and V, can be explained in terms of band filling induced by introduction of V atoms in YB₂. Within the range of temperatures (0–1200 K) and pressures (0–15 GPa) studied, the band-filling effect is found to result in significant positive deviations in the values of shear strength, stiffness, and hardness of superlattice-structured Y_0.5V_0.5B₂ from those evaluated from its constituent compounds using the Vegard’s law, respectively, by $\sim$8%, $\sim$5%, and $\sim$25%, and the hardness of superlattice-structured Y_0.5V_0.5B₂ is $\sim$40 GPa potentially indicating its superhard nature. These consequences strongly underline the essential impact of band filling on improvement in mechanical behavior and stability of YB₂ through alloying with VB₂, and also they can be served as guidance for further advancement of hard-coating technology based especially on transition-metal diborides.