Binary alloys composed of two fcc metals but featuring a bcc ground state.

Certain binary alloys, composed of two face-centered cubic (fcc) metals, have a body-centered cubic (bcc) ground state structure under ambient conditions. The reason behind this counterintuitive observation is explored and discussed in detail here. 36 binary alloys consisting of fcc metals (nine transition metals in groups 9, 10 and 11: Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) have been evaluated. Nine binary alloys (PdCu, AuCu, AuNi, AgNi, IrCu, RhCu, CuNi, PdNi, and PdCo) are screened to have the bcc phase as the ground state, among which, only PdCu has readily accessible fcc and bcc phases in terms of cohesive energy. A simple two-body interaction model (the 2M-kNN model) in combination with Monte Carlo (MC) simulations provides an electronic level understanding of the relative stability of the PdCu alloy in both fcc and bcc structures, as well as its fcc-to-bcc phase transition. The effective repulsion among the 1NN (first nearest neighbor) atom pairs of the same element is the driving force to afford thermodynamically favored ordered fcc PdCu alloy structures instead of the disordered ones. At nearly equal atom ratio, the repulsion cannot be avoided much, and the ordered fcc ground state configuration experiences anisotropic strain. The directional lattice expansion and compression partially relieve the 1NN repulsion from homo diatomic pairs and simultaneously reduce the distance between attractive hetero diatomic pairs. When the anisotropic strain is sufficiently large (around equal atom ratio), the atom shear transformations lead to the phase transition. This study can be further extended to cover other types of metals and binary alloys.