Interface assisted formation of a metastable hcp phase by ion mixing in an immiscible Ag-Ni system

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in the Ag80Ni20 multilayered films, in which the excess interfacial free energy provided partial driving force for alloying in an equilibrium immiscible Ag-Ni system characterized by a large positive heat of formation. The metastable phase was identified to be of D0(19) hcp structure with a stoichiometry of Ags Ni and can therefore be considered as a Hume-Rothery 7/4 electron compound. A thermodynamic calculation showed that when the multilayers consisted of 8 or more bilayers, the interfacial free energy could elevate their initial energy levels up to a state higher than that of the hcp phase, and that while the multilayers composed of 6 or less bilayers, it was thermodynamically not favoured to form the hcp phase. Kinetically, the metastable hcp phase was grown from the fee Ag lattice through a fast sliding mechanism. Furthermore, an ab initio calculation showed that a minimum total energy of the Ag3Ni phase did correspond to the above observed metastable state, indicating that the stability of the metastable Ag-Ni hcp phase was correlated with its electronic structure.