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 S1like ordering tendency will form. Previous short-range order diffraction data are quantitatively consistent with these new predictions.
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