Incomplete melting of the Au(100) surface

Thermodynamic States in Nonhomogeneous Systems: From Nanoscale to Macroscale

We analyze the mechanisms leading to thermodynamic stable states and isobaric phase transitions in finite nonhomogeneous nanosystems using classical molecular dynamics. We consider systems ranging from nano- to macroscopic scales and focus on spherical Lennard-Jones nanoparticles, in both one- and two-phase equilibria. In particular, we investigate how these systems' macroscopic behaviors evolve as their size increases. Our findings unveil that nonhomogeneous stable states are governed by spatial variations in intensive variables, contrary to standard thermodynamics of homogeneous systems, where equilibrium is described by extensive variables. Crucially, we demonstrate that nonhomogeneous intensive variables persistently diverge from homogeneous systems' predictions, even as the system size increases. Our calculations show that one-phase equilibrium is the direct consequence of the spatial variations of these intensive variables. In the two-phase equilibrium, such variations generate isobaric phase transitions across finite temperature intervals, through a continuous sequence that includes three-phase states. These temperature ranges do not vanish with increasing size, challenging the assumption that homogeneous systems are the asymptotic limit of finite nonhomogeneous systems. Our findings highlight the significance of boundary effects in understanding thermodynamic stability and equilibrium mechanisms, marking a departure from standard thermodynamic models that neglect these variations.

Size-dependent melting modes and behaviors of Ag nanoparticles: a molecular dynamics study

The size-dependent melting behaviors and mechanisms of Ag nanoparticles (NPs) with diameters of 3.5-16 nm were investigated by molecular dynamics (MD). Two distinct melting modes, non-premelting and premelting with transition ranges of about 7-8 nm, for Ag NPs were demonstrated via the evolution of distribution and transition of atomic physical states during annealing. The small Ag NPs (3.5-7 nm) melt abruptly without a stable liquid shell before the melting point, which is characterized as non-premelting. A solid-solid crystal transformation is conducted through the migration of adatoms on the surface of Ag NPs with diameters of 3.5-6 nm before the initial melting, which is mainly responsible for slightly increasing the melting point of Ag NPs. On the other hand, surface premelting of Ag NPs with diameters of 8-16 nm propagates from the outer shell to the inner core with initial anisotropy and late isotropy as the temperature increases, and the close-packed facets {111} melt by a side-consumed way which is responsible for facets {111} melting in advance relative to the crystallographic plane {111}. Once a stable liquid shell is formed, its size-independent minimum thickness is obtained, and a three-layer structure of atomic physical states is set up. Lastly, the theory of point defect-pair (vacancy-interstitial) severing as the mechanism of formation and movement of the solid-liquid interface was also confirmed. Our study provides a basic understanding and theoretical guidance for the research, production and application of Ag NPs.

Nonhomogeneous surface premelting of Au nanoparticles

A reversible nonhomogeneous surface premelting of Au nanoparticles is demonstrated through molecular dynamics simulations. With increasing temperature, liquid-like atoms first appear at some vertices and edges of surface facets, then small liquid regions grow and, at temperatures close to the particle melting temperature, most of the remaining solid-like surface atoms reside on {111} planes which are the most stable against surface premelting. The appearance of a contiguous liquid layer (complete surface premelting) is size dependent and is not observed in very small nanoparticles.