Stability Of The Non-equilibrium Binary Alloy Phase Studied By Lattice Dynamics

Firstly, a brief review is presented concerning the metastability of the non-equilibrium phases, the many-body potential, the first-principles, and the lattice dynamics calculation methods.Secondly, the metastability of the non-equilibrium phases in Co-Ag, Ag-Ru, and Co-Au systems are studied. The tight-binding potentials are used in the Co-Ag and Ag-Ru system, combined with first-principles calculations. Through comparison of the cohesion energy, elastic constant, and phonon spectra, the metastability of the non-equilibrium phases are clarified. While some of the metastable phases are indeed observed in ion beam mixing experiments. Furthermore, analysis of the spatial valence charge density indicates that a specific phase/structure can only be stabilized, when there are high enough spatial valence charge density, corresponding to large enough electronic interactions, between the similar atoms to form the relevant bonding of the phase/structure. Besides, through first-principles calculation with Co-Au system, we find that the spin density may also play an important role in influencing the structural stability of the ferromagnetic metastable phases.Thirdly, high-pressure metastable phases in Cu-Ta and Co-Cu systems are studied. Based on an embedded atom method potential, we find that the minimum energy states of the three ideal lattices, i.e. the L12, D03 and D019 structures, of the Cu3Ta compounds are all unstable at zero pressure, as there appear many associated imaginary phonons, and that, by raising the pressure up to the respective critical values, the three ideal lattices can all be stabilized while the imaginary phonons completely disappear. Besides, through first-principles calculation, we find that the immiscible Co-Cu compound can also be stabilized under high pressure. These are consisted with the very recent proposed concept of high-pressure alloying between immiscible elements. In addition, an interesting oscillation behavior of the phonon spectra of the L12 CoCu3 compound under different pressures is observed, showing the oscillation behavior of the metastability of the CoCu3 compound.Fourthly, nine 16-atom special quasirandom structures (SQS) unit cells (in which six of them are our newly developed), are used to mimic the random binary fcc substitutional solid solutions. A new error analysis method for the comparison of the different composition with the same number of atoms in SQS unit cell is also proposed. These developed SQS are general and can be easily applied to study substitutional disordered alloys.In the end, these SQS are then applied in first-principles calculation to predict the lattice constants, heats of formation, and magnetic moments of the CoxCu1-x solid solutions and the calculated results are in good agreement with the experimental data, and other theoretical calculations, when available. Especially the sharp drop of the magnetic moments of the CoxCu1-x solid solutions around composition x=0.1 are well reproduced in present first-principles calculation. Furthermore, the obtained charge densities show that in the CoxCu1-x solid solutions, the valence charge mostly distribute between the Co-Co atoms, forming the attractive covalent bonding.