First-principles prediction of crystal structures and mechanical properties in Nb ₂ B ₅

By integrating the CALYPSO structure prediction technique with first-principles calculations, we systematically explore the crystal structures of Nb2B5, with five known prototype compounds (Zr2B5, V2B5, and W2B5) included as candidate structures. The computational results demonstrate that the C2/m phase is the energetically most favourable structure under ambient pressure, whereas the alternative Zr2B5-type structure emerges as the preferred phase under high-pressure environments. Comprehensive assessments of the formation enthalpies, AIMD simulations, phonon dispersion curves, and elastic constants confirm that these two phases are thermodynamically, dynamically, and mechanically stable. The large bulk moduli and high hardness values derived for both phases underscore their exceptional incompressibility and outstanding mechanical rigidity. Moreover, the elastic anisotropic behaviour is quantified through appropriate anisotropy indices along with three- and two-dimensional graphical representations of the Young’s modulus. The detailed electronic structure and chemical bonding investigations reveal that the robust covalent B-B and Nb-B interactions are the primary contributors to the exceptional stability and hardness exhibited by both phases. These theoretical findings are expected to facilitate further experimental research on the synthesis of such novel materials.