Ab initio calculations of the structure and electronic density of states (DOS) of the perfect core-shell ${\mathrm{Ag}}_{27}{\mathrm{Cu}}_7$ nanoalloy attest to its $D_{5h}$ symmetry and confirm that it has only six nonequivalent (two Cu and four Ag) atoms. The analyses of bond length, average formation energy, and heat of formation of ${\mathrm{Ag}}_{27}{\mathrm{Cu}}_7$ and $L{1}_2$ bulk Ag-Cu alloys provide an explanation for the relative stability of the former with respect to the other nanoalloys in the same family. The highest occupied molecular orbital–lowest unoccupied molecular orbital gap is found to be $0.77\mathrm{eV}$, which is in agreement with previous results. The analyses of the DOS of ${\mathrm{Ag}}_{27}{\mathrm{Cu}}_7$, $L{1}_2$ Ag-Cu alloys, and related systems provide insight into the effects of low coordination, contraction or expansion, and the presence of foreign atoms on the DOS of Cu and Ag. While some characteristics of the DOS are reminiscent of those of the phonon-stable $L{1}_2$ Ag-Cu alloys, the Cu and Ag states hybridize significantly in ${\mathrm{Ag}}_{27}{\mathrm{Cu}}_7$, compensating for the $d$-band narrowing that each atom undergoes and hindering the dip in the DOS found in the bulk alloys. Charge density plots of ${\mathrm{Ag}}_{27}{\mathrm{Cu}}_7$ provide further insight into the relative strengths of the various interatomic bonds. Our results for the electronic and geometric structures of this nanoalloy can be explained in terms of length and strength hierarchies of the bonds, which may have implications also for the stability of alloys in any phase or size.

• Received 14 December 2007

DOI:https://doi.org/10.1103/PhysRevB.77.195404