Atomic-scale insights into electronic, elastic properties and superconductivity in Nb-based A15 compounds

Abstract Nb-based A15 superconductors are indispensable for high-field applications due to their exceptional superconducting properties, yet the atomic-scale origins of their performance require further elucidation. Through comprehensive first-principles calculations on the Nb 3 X series (X = Al, Ga, Ge, In, Sn, Pb), we establish a unified picture linking composition, electronic structure, mechanical properties, and superconductivity. The electronic structure is metallic and dominated by Nb-4 d states, featuring prominent van Hove singularities (VHS) near the Fermi level. Crucially, anisotropic splitting of the Nb- t 2 g orbitals differentiates the conductive Nb–Nb chain direction from the non-chain directions. The VHS, concentrated along the Nb–Nb chains, provides a high density of states (DOS) that is a key electronic driver for strong electron–phonon coupling (EPC). Systematic elastic property analysis reveals that hardness and shear modulus increase from Group IIIA to Group IVA elements, a trend attributed to enhanced covalent bonding with the additional valence electron. Consequently, Nb 3 Ge exhibits the highest shear modulus and hardness, while Nb 3 Al shows the strongest elastic anisotropy. For the prototypical Nb 3 Ge, the coupling between the high-DOS electronic states and a characteristic soft phonon modes associated with Nb–Nb chain vibrations yields a large EPC strength λ = 1.83 and a critical temperature T C = 23.33 K, in excellent agreement with experiment. This systematic study demonstrates how valence electron count, atomic size, and bonding character collectively govern the electronic, mechanical, and superconducting properties in A15 Nb 3 X compounds, providing critical insights for the rational design of high-performance superconductors.