First-principles study of electronic properties and stability of Nb ₅SiB₂ (001) surface

The density functional calculations are performed to study the electronic structure and stability of Nb₅SiB₂ (001) surface with different terminations. The calculated cleavage energies along the (001) planes in Nb₅SiB₂ are 5.015 J m^-2 and 6.593 J m ^-2 with the break of Nb - Si and Nb - NbB bonds, respectively. There exists a close correlation between the surface relaxation including surface ripple and the cleavage energy: the larger the cleavage energy, the larger the surface relaxation. Moreover, the surface stability of the Nb ₅SiB₂ (001) with different terminations has been investigated by the chemical potential phase diagram. Prom a thermodynamics point of view, the four terminations can be stabilized under different conditions. In chemical potential space, NbB (Nb) and Nb (Si) terminations are just stable in a small area, whereas Si (Nb) and Nb (NbB) terminations are stable in a large area (the letters in brackets represent the subsurface atoms).