Phenomenology of the heating, melting and diffusion processes in Au nanoparticles

The paper reports the results of a molecular dynamics study of the heating and melting process of nanoparticles with 1985 to 84 703 atoms. Building on a previous study by the present authors [Bertoldi, et al., J. Phys. Chem. Solids, 2017, 111, 286-293] involving the energy versus temperature, the Lindemann index and the radial distribution function, the current work relies on the mean-square displacement, the Lindemann ratio and the simulated snapshots to characterize four regions in the process of heating-to-melting. A general pattern of the atomic configuration evolution upon heating and a systematics of the transition temperatures between the various identified steps, is proposed. In addition, the most significant, so-called "melting step" in this process is analyzed in terms of the quasi-chemical approach proposed by Bertoldi et al., which treats this step by invoking a dynamic equilibrium of the type Au (LEA-SPL) Au (HEA-LPL) involving low-energy atoms (LEA) and high-energy atoms (HEA) forming the solid phase-like (SPL) and the liquid phase-like (LPL) states of the system, respectively. The "melting step" is characterized by evaluating the equal-Gibbs energy temperature, i.e., the "T-0 temperature", previously introduced by the current authors, which is the thermodynamic counterpart of the temperature of fusion of macroscopic elemental solids. The diffusion coefficients at T-0 are determined, and their spatial and temperature dependence is discussed. In particular, the activation energy for the atom movements in the HEA-LPL/LEA-SPL mixture at T-0 is reported. The consistency between the current phenomenological picture and microscopic interpretation of the thermodynamic, kinetic and atomic configuration information obtained is highlighted.