Role of string-like collective atomic motion on diffusion and structural relaxation in glass forming Cu-Zr alloys

Abstract

We investigate Cu-Zr liquid alloys using molecular dynamics simulation and well-accepted embedded atom method potentials over a wide range of chemical composition and temperature as model metallic glass-forming (GF) liquids. As with other types of GF materials, the dynamics of these complex liquids are characterized by "dynamic heterogeneity" in the form of transient polymeric clusters of highly mobile atoms that are composed in turn of atomic clusters exhibiting string-like cooperative motion. In accordance with the string model of relaxation, an extension of the Adam-Gibbs (AG) model, changes in the activation free energy ΔGa with temperature of both the Cu and Zr diffusion coefficients D, and the alpha structural relaxation time τα can be described to a good approximation by changes in the average string length, L. In particular, we confirm that the strings are a concrete realization of the abstract "cooperatively rearranging regions" of AG. We also find coexisting clusters of relatively "immobile" atoms that exhibit predominantly icosahedral local packing rather than the low symmetry packing of "mobile" atoms. These two distinct types of dynamic heterogeneity are then associated with different fluid structural states. Glass-forming liquids are thus analogous to polycrystalline materials where the icosahedrally packed regions correspond to crystal grains, and the strings reside in the relatively disordered grain boundary-like regions exterior to these locally well-ordered regions. A dynamic equilibrium between localized ("immobile") and wandering ("mobile") particles exists in the liquid so that the dynamic heterogeneity can be considered to be type of self-assembly process. We also characterize changes in the local atomic free volume in the course of string-like atomic motion to better understand the initiation and propagation of these fluid excitations.

Stokes-Einstein relation of the liquid metal rubidium and its relationship to changes in the microscopic dynamics with increasing temperature

“…Schober pointed out that collective motions are at the origin of the breakdown of the SE relation there [68]. In a MD simulation study on a CuZr liquid alloy the observed dynamic heterogeneity was linked to collective stringlike motions and this suggestion supports the view that collective motions are at the center for changes in the dynamics [69]. Our data on the microscopic structural relaxation time of density fluctuations (see Fig.…”

Superionic UO2: A model anharmonic crystalline material

“…The strings are clearly polymer-like in form and polydisperse in length, as found before repeatedly in glass-forming liquids, 19,28,29,100 and we determine the length distribution of the strings and their fractal geometry. The resulting size distribution of the strings found in our equilibrium UO2 samples is indicated in Figure 15 where we find that the distribution above T  is exponential to a good approximation, again as generally found in all glass-forming liquids studied to date.…”

“…While these simulation results are highly suggestive, they do not support a coherent picture of relaxation and diffusion that was found before in cooled liquids. 28,29 The current work focuses on UO2 as a model superionic material for which there are excellent potentials and corresponding measurement studies of diverse thermodynamic and transport properties in view of its high practical interest, 15,16 and we analyze this model material based on a methodology used previously to quantify the dynamics of glass-forming liquids to determine what features are common and which features are specific to superionic crystalline materials, as compared to the glassy dynamics of cooled liquids. We are motivated to study this problem also as a model material for better understanding ionic transport in amorphous superionic materials, which exhibit many phenomenological parallels with superionic crystalline materials and promising superionic Li salt materials in the development of safer and longer lasting battery materials.…”

Temperature-dependent structure evolution in liquid gallium

“…The cooperative dynamics analysis based on the atomic motion, is to quantify the collective atomic displacement motion [81]. Firstly, we define the mobile atoms by comparing the self- where  is the half distance of the average atomic distance, mobile atoms i and j are spatially considered to be collective relations.…”