Reinterpreting the Cu-Pd phase diagram based on new ground-state predictions

Our notions of the phase stability of compounds rest to a large extent on the experimentally assessed phase diagrams. Long ago, it was assumed that in the Cu-Pd system for x(Pd) <= 25% there are at least two phases at high temperature (L1(2) and a L1(2)-based superstructure), which evolve into a single L1(2)-ordered phase at low temperature. By constructing a first-principles Hamiltonian, we predict a yet undiscovered Cu(7)Pd ground state at x(Pd) = 12.5% (referred to as S1 below) and an L1(2)-like Cu(9)Pd(3) superstructure at 25% (referred to as S2). We find that in the low-temperature regime, a single L1(2) phase cannot be stable, even with the addition of anti-sites. Instead we find that an S2-phase with S1-like ordering tendency will form. Previous short-range order diffraction data are quantitatively consistent with these new predictions.