Elastic properties of superconducting NbB2+x obtained from first-principles calculations

Abstract

First-principles calculations have been performed on NbB2+x to obtain its elastic constants using the plane wave pseudopotential method based on density functional theory, within the generalized gradient approximation. We have derived the bulk moduli, shear moduli, Young's moduli, Poisson's ratios and elastic anisotropies for NbB2+x polycrystalline aggregates. The elastic properties obtained here show that the structure becomes less symmetrical as the boron content increases. The higher elastic anisotropies A(K) and A(G) were associated with the higher superconducting transition temperature T-c reported for this material (Escamilla et al 2004 J. Phys.: Condens. Matter 16 5979). Using the Debye temperature, theta(D), and T-c, the electron-phonon coupling constant. was obtained from the McMillan equation. For the concentrations from (B/Nb) = 2.20(1) to 2.34(1), the values of lambda increase from 0.497 to 0.572

this suggests that NbB2+x is a weakly coupled Bardeen-Cooper-Schrieffer type superconductor.