Electronic structure, chemical bonding, phase stability, and ground-state properties ofYNi2−x(Co/Cu)xB2C

In order to understand the role of Ni site substitution on the electronic structure and chemical bonding in ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ we have made systematic electronic-structure studies on ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as a function of Co and Cu substitution using the supercell approach within the local density approximation. The equilibrium volume, bulk modulus ${(B}_{0})$ and its pressure derivative ${(B}_{0}^{\ensuremath{'}}),$ Gr\"uneisen constant $({\ensuremath{\gamma}}_{G}),$ Debye temperature $({\ensuremath{\Theta}}_{D}),$ cohesive energy ${(E}_{c}),$ and heat of formation $(\ensuremath{\Delta}H)$ are calculated for ${\mathrm{YNi}}_{2\ensuremath{-}x}(\mathrm{C}\mathrm{o}/\mathrm{C}\mathrm{u}{)}_{x}{\mathrm{B}}_{2}\mathrm{C}$ $(x=0,0.5,1.0,1.5,2).$ From the total energy, electron-energy band structure, site decomposed density of states, and charge-density contour we have analyzed the structural stability and chemical bonding behavior of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as a function of Co/Cu substitution. We find that the simple rigid band model successfully explains the electronic structure and structural stability of Co/Cu substitution for Ni. In addition to studying the chemical bonding and electronic structure we present a somewhat speculative analysis of the general trends in the behavior of critical temperature for superconductivity as a function of alloying.