Allotropic phenomena in silver metals: Search for hexagonal phases, structure determination, and thermodynamic stability

Existing physics generally suggests that, unlike metals such as Fe and Ti, pure Ag, which is a common metal, has only one crystal structure, face-centered cubic, and no microstructural evolution. In this study, we proposed a unique method to find and confirm the hexagonal phase in Ag, including rapid cooling and TEM characterization, and confirmed that pure Ag undergoes an isomeric transition from a face-centered cubic (FCC) crystal structure to a hexagonal-close packed (HCP) crystal structure at elevated temperatures from 400 °C to 500 °C. In addition, the Gibbs free energy of silver`s face-centered cubic and hexagonal close-packed crystal structures within the temperature range of 300 K to 900 K was calculated using non-equilibrium molecular dynamics (MD) and GIBBS2. The calculation results indicate that the Gibbs free energy difference between the fcc and hcp phases of Ag is minimal. In light of this, the phonon spectra were also calculated in this study to elucidate the kinetic transformation mechanism, further demonstrating the occurrence of structural transformation. Finally, the results show that the densely-arranged hexagonal phase obtained from the isomeric transition of Ag at high temperatures in this study can be retained up to room temperature, which implies that it is feasible to obtain bulk hexagonal phases based on the phase transformation due to the isomeric transition to change the physical and mechanical properties of Ag metal by controlling the cooling rate, for example.