Boron excess and external pressure effects on NbB2

We report structural and electronic studies on NbB$_{2+x }$with nominal compositions 2.0 $\le $ B/Nb $\le $ 2.6. After refinement the obtained compositions were found that are superconducting for (B/Nb)$_{ref} \quad \ge $ 2.20(2). Structural analyses reveal that the boron excess induces significant changes in the Nb-B bond length, giving rise to an increase in the $c$ axis and in the cell volume. The superconducting transition temperature, T$_{C}$, increases with boron content to a maximum at 9.8 K, for the composition (B/Nb)$_{ref}$ = 2.34(1). Electrical resistance measurements as a function of temperature and pressure show that T$_{C}$ decreases at different rates, depending on boron excess. Samples with high concentration of boron show low dTc/dP rates. These experimental results are discussed in terms of possible Nb vacancies. The changes of T$_{C}$ are correlated with the evolution of the structural parameters and compared with theoretical predictions of band structure.