We show that the existence of stable, ordered $3d-5d$ intermetallics CuAu and NiPt, as opposed to the unstable $3d-4d$ isovalent analogs CuAg and NiPd, results from relativity. First, in shrinking the equilibrium volume of the $5d$ element, relativity reduces the atomic size mismatch with respect to the $3d$ element, thus lowering the elastic packing strain. Second, in lowering the energy of the bonding $6s,p$ bands and raising the energy of the $5d$ band, relativity enhances (diminishes) the occupation of the bonding (antibonding) bands. The raising of the energy of the $5d$ band also brings it closer to the energy of the $3d$ band, improving the $3d-5d$ bonding.

• Received 19 June 2002

DOI:https://doi.org/10.1103/PhysRevB.67.092103