Diatomic substitutionals in superconducting Nb(1−x)B2

Using first-principles, density functional theory calculations, we have shown that the increase in superconducting critical temperature to above 9 K experimentally observed for B-rich, off-stoichiometry NbB2 material is associated with the formation of B-dimers on Nb sites rather than with the formation of Nb vacancies, as previously proposed. Our calculations show that certain of these B-dimer configurations have lower binding energies than do the vacancy structures. This result is pressure independent in the range between 0 and 10 GPa, and above. Further, the dimer-containing materials have a higher electronic density of states at the Fermi level than do the vacancy structures. Finally, the presence of B-dimers results in the splitting off of phonon normal modes. These last two factors are relevant to the improved superconducting properties.