Structural relaxation of nanocrystalline PdAu alloy: Mapping pathways through the potential energy landscape

Abstract

Preparation history and processing have a crucial influence on which configurational state material systems assume. Glasses and nanocrystalline materials usually reside in nonequilibrium states at room temperature, and as a consequence, their thermodynamic, dynamical, and physical properties change with time—even years after manufacture. Such changes, entitled aging or structural relaxation, are all manifestations of paths taken in the underlying potential energy landscape. Since it is highly multidimensional, there is a need to reduce complexity. Here, we demonstrate how to construct a one-dimensional pathway across the energy landscape using strain/volume as an order parameter. On its way to equilibrium, we map the system’s release of energy by calorimetry and the spectrum of barrier heights by dilatometry. The potential energy of the system is reduced by approximately kBT during relaxation, whereas the crossing of saddle points requires activation energies in the order of 1eV/atom relative to the energy minima. As a consequence, the system behaves as a bad global minimum finder. We also discovered that aging is accompanied by a decrease in the non-ergodicity parameter, suggesting a decline in density fluctuations during aging.