New alloys: mixing it up on metal surfaces.

Normally, gold and nickel don't mix to create an alloy. The atoms of these two elements are sufficiently different in size that they tend to remain segregated.

However, researchers have discovered recently that pairs of incompatible elements can readily mix to form an alloy - as long as the mixture is confined to a single layer of atoms on the surface of a crystal of one of the two metals. These novel, two-dimensional alloys include mixtures of gold and nickel on a nickel surface, silver and platinum, antimony and silver, and sodium or potassium and aluminum.

"The fact that you can form an alloy at a surface of something that won't form one in the bulk is interesting and striking," says Jerry D. Tersoff of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. "It gives you the opportunity for creating materials that couldn't otherwise exist."

Now, Tersoff proposes that such surface alloys are likely to form whenever a mismatch of atomic sizes is the main factor determining the behavior of a mixture of two elements. He describes his theoretical argument in the Jan. 16 PHYSICAL REVIEW LETTERS.

"This explains why the phenomenon is surprisingly common," Tersoff says. Remarkably, the same effect that suppresses intermingling within a crystal - atomic size mismatch - favors intermixing at its surface.

Mixing atoms of different sizes creates mechanical strains in the material, and the atoms tend to arrange themselves to minimize the strain energy. Tersoff's calculations and computer simulations show that on a surface, mixing represents a good strategy for reducing the strain.

For example, to minimize strain, gold atoms resting on a nickel surface tend to stay as far apart from each other as possible (see illustration) [ILLUSTRATION OMITTED]. This allows nickel atoms to fill in the gaps to create a surface alloy.

These alloys may prove useful in processes such as catalysis.