First-principles predictions of yet-unobserved ordered structures in the Ag-Pd phase diagram.

The complexity of first-principles total energy calculations limits the pool of structure types considered for a ground-state search for a binary alloy system to a rather small, O(10), group of "usual suspects." We conducted an unbiased search of fcc-based Ag(1)-(x)Pd(x) structures consisting of up to many thousand atoms by using a mixed-space cluster expansion. We find an unsuspected ground state at 50%-50% composition-the L1(1) structure, currently known in binary metallurgy only for the Cu(0.5)Pt(0.5) alloy system. We also provide predicted short-range-order profiles and mixing enthalpies for the high temperature, disordered alloy.