Effect of pentagonal-coordinated surface on crystal nucleation of an undercooled melt.

Bringing a liquid into contact with a solid is known to generally promote crystal nucleation at the freezing temperature. In contrast, it is much more difficult to conceive that a solid surface may hinder nucleation and favor large undercooling effects. Here we report on ab initio and classical molecular dynamic simulations to capture the underlying structural mechanism responsible for this striking effect. We find that the substrate/liquid interactions exert an important influence on in-plane ordering of the adjacent liquid layers in the undercooling regime. In particular, we identify that the presence of atomic arrangements with five-fold symmetry (FFS) on the substrate surface in the form of pentagonal atomic motifs allows the liquid to be undercooled well below its freezing temperature. Our findings clearly demonstrate that this pentagonal-coordinated surface enhances the presence of local arrangements with FFS in the adjacent liquid layers that prevents the crystal nucleation. Finally we suggest new technological developments to attain large undercooling effects.

Communication: Fast dynamics perspective on the breakdown of the Stokes-Einstein law in fragile glassformers.

The breakdown of the Stokes-Einstein (SE) law in fragile glassformers is examined by Molecular-Dynamics simulations of atomic liquids and polymers and consideration of the experimental data concerning the archetypical ortho-terphenyl glassformer. All the four systems comply with the universal scaling between the viscosity (or the structural relaxation) and the Debye-Waller factor ⟨u2⟩, the mean square amplitude of the particle rattling in the cage formed by the surrounding neighbors. It is found that the SE breakdown is scaled in a master curve by a reduced ⟨u2⟩. Two approximated expressions of the latter, with no and one adjustable parameter, respectively, are derived.

Dynamical, structural and chemical heterogeneities in a binary metallic glass-forming liquid.

As it approaches the glass transition, particle motion in liquids becomes highly heterogeneous and regions with virtually no mobility coexist with liquid-like domains. This complex dynamic is believed to be responsible for different phenomena including non-exponential relaxation and the breakdown of the Stokes-Einstein relation. Understanding the relationships between dynamical heterogeneities and local structure in metallic liquids and glasses is a major scientific challenge. Here we use classical molecular dynamics simulations to study the atomic dynamics and microscopic structure of [Formula: see text] alloy in the supercooling regime. Dynamical heterogeneities are identified via an isoconfigurational analysis. We demonstrate the transition from isolated to clustering low mobility with decreasing temperature. These slow clusters, whose sizes grow upon cooling, are also associated with concentration fluctuations, characterized by a Zr-enriched phase, with a composition [Formula: see text]. In addition, a structural analysis of slow clusters based on Voronoi tessellation evidences an increase with respect of the bulk system of the fraction of Cu atoms having a local icosahedral order. These results are in agreement with the consolidated scenario of the relevant role played by icosahedral order in the dynamic slowing-down in supercooled metal alloys.

How closely do many-body potentials describe the structure and dynamics of Cu-Zr glass-forming alloy?

Molecular dynamics investigations of the structure and dynamics of Cu64.5Zr35.5 metallic glass-forming alloy have been carried out using five different semi-empirical, many-body interaction potentials based on the Finnis-Sinclair model [M. I. Mendelev et al., J. Appl. Phys. 102, 043501 (2007) (MSK); M. I. Mendelev et al., Philos. Mag. 89, 967 (2009) (MKOSYP); L. Ward et al., e-print arXiv:1209.0619 (2012) (WAFW)] and the embedded-atom model [Y. Q. Cheng et al., Phys. Rev. Lett. 102, 245501 (2009) (CMS) and N. Jakse et al., Phys. Rev. B 85, 174201 (2012) (JNP)]. Although the total static structure factor of the alloy for all the five interaction potentials is, in general, found to be in good agreement with the experimental results, the investigation of a local structure in terms of icosahedral short-range order reveals that the effect of the interaction potential (especially the cohesive part) on the structure of the alloy is not as trivial as it seems. For MSK and JNP potentials, the self-intermediate scattering function Fs(q, t), q-dependence of the structural relaxation time τα in the low-q region, and the self-diffusion coefficient, Ds, for Cu-atoms in the alloy are in excellent agreement with the experimental results. The results for MKOSYP, CMS, and WAFW potentials deviate significantly from the experiment and suggest the dynamics of the alloy to be faster. The difference in the description of the dynamics of the alloy by different potentials is found to be due to the difference in the relevant energy scales corresponding to the temperature scales. τα and Ds exhibit Arrhenius temperature dependence in the high temperature regime above the melting temperature. We also suggest that the attractive forces influence the dynamics of the liquid alloy significantly, which is against the mere perturbative role assigned to the attractive forces in the van der Waals picture of liquids that has been challenged in the recent years. As the five interaction potentials are frequently employed to study thermodynamic, mechanical, and transport properties of Cu-Zr alloys, our study also provides a suitability check for these potentials.

Striking impact of Na insertion on structural and electronic properties of the electrode material Na2+x V6O16.

The Na2.55V6O16 structure is a promising material for sodium ion batteries due to a significant capacity and stability at high current rates, but its cycle stability for Na application is significantly lower than that obtained with Li (de-)insertion. In this work, we present the results of density functional theory (DFT) calculations on the structural and electronic properties of Na2+x V6O16 compounds up to x = 2. As x increases, we evidence a strong deformation of the tetrahedral sites occupied by sodium ions, leading to new highly stable sites for these inserted ions at x = 2. Comparing with Li4V6O16, we demonstrate that the stability of these new Na sites can be attributed to the electrostatic interactions between sodium ions and atoms of the host structure, the evolution of the oxidation degree of Vanadium atoms being another indicator of such effects.

Validity of the Stokes-Einstein relation in liquids: simple rules from the excess entropy.

It is becoming common practice to consider that the Stokes-Einstein relation D/T~ η -1 usually works for liquids above their melting temperatures although there is also experimental evidence for its failure. Here we investigate numerically this commonly-invoked assumption for simple liquid metals as well as for their liquid alloys. Using ab initio molecular dynamics simulations we show how entropy scaling relationships developed by Rosenfeld can be used to predict the conditions for the validity of the Stokes-Einstein relation in the liquid phase. Specifically, we demonstrate the Stokes-Einstein relation may break down in the liquid phase of some liquid alloys mainly due to the presence of local structural ordering as evidenced in their partial two-body excess entropies. Our findings shed new light on the understanding of transport properties of liquid materials and will trigger more experimental and theoretical studies since excess entropy and its two-body approximation are readily obtainable from standard experiments and simulations.

Transport properties and Stokes-Einstein relation in Al-rich liquid alloys.

We use ab initio molecular dynamics simulations to study the transport properties and the validity of the Stokes-Einstein relation in Al-rich liquid alloys with Ni, Cu, and Zn as alloying elements. First, we show that the composition and temperature dependence of their transport properties present different behaviors, which can be related to their local structural ordering. Then, we evidence that the competition between the local icosahedral ordering and the local chemical ordering may cause the breakdown of the Stokes-Einstein relation even in the liquid phase. We demonstrate that this breakdown can be captured by entropy-scaling relationships developed by Rosenfeld and using the two-body excess entropy. Our findings provide a unique framework to study the relation between structure, thermodynamics, and dynamics in metallic melts and pave the way towards the explanation of various complex transport properties in metallic melts.

Excess Entropy Scaling Law for Diffusivity in Liquid Metals.

Understanding how dynamic properties depend on the structure and thermodynamics in liquids is a long-standing open problem in condensed matter physics. A very simple approach is based on the Dzugutov contribution developed on model fluids in which a universal (i.e. species-independent) connection relates the pair excess entropy of a liquid to its reduced diffusion coefficient. However its application to "real" liquids still remains uncertain due to the ability of a hard sphere (HS) reference fluid used in reducing parameters to describe complex interactions that occur in these liquids. Here we use ab initio molecular dynamics simulations to calculate both structural and dynamic properties at different temperatures for a wide series of liquid metals including Al, Au, Cu, Li, Ni, Ta, Ti, Zn as well as liquid Si and B. From this analysis, we demonstrate that the Dzugutov scheme can be applied successfully if a self-consistent method to determine the packing fraction of the hard sphere reference fluid is used as well as the Carnahan-Starling approach to express the excess entropy.

Electrochemical and ab initio investigations to design a new phenothiazine based organic redox polymeric material for metal-ion battery cathodes.

Different N-substituted phenothiazines have been synthesized and their electrochemical behavior has been investigated in CH3CN in order to design the best polyphenothiazine based cathodic material candidate for lithium batteries. These compounds exhibit two successive reversible one-electron oxidation processes. Ab initio calculations demonstrate that the potential of the first process is a result of both the hybridization effects between the substituent and the phenothiazine unit as well as the change of conformation of the phenothiazine heterocycle during the oxidation process. More specifically, we show that an asymmetric molecular orbital spreading throughout an external cycle of the phenothiazine unit and the alkyl fragment is formed only if the alkyl fragment is long enough (from the methyl moiety onwards) and is at the origin of the bent conformation for N-substituted phenothiazines during oxidation. Electrochemical investigations supported by ab initio calculations allow the selection of a phenothiazinyl unit which is then polymerized by a Suzuki coupling strategy to avoid the common solubilization issue in carbonate-based liquid electrolytes of lithium cells. The first electrochemical measurements performed show that phenothiazine derivatives pave the way for a promising family of redox polymers intended to be used as organic positives for lithium batteries.

Relationship between structure and dynamics in liquid Al1-xNix alloys.

Using ab initio molecular dynamics simulations, we study structural and dynamic properties of liquid Al-Ni alloys at T = 1795 K as a function of concentration. From partial pair-correlation functions and Bhatia-Thornton structure factors, we show a non-linear composition dependence of chemical short-range order. Additional analysis based on three-dimensional pair-analysis techniques evidences a strong interplay between icosahedral short-range order and chemical short-range order in the Ni-rich range. In examining transport properties like self-diffusion and inter-diffusion coefficients, we demonstrate a close relationship between the structural changes and the non-linear evolution of dynamic properties as a function of composition.

Correlation between dynamic slowing down and local icosahedral ordering in undercooled liquid Al80Ni20 alloy.

We use ab initio molecular dynamics simulations to study the correlation between the local ordering and the dynamic properties of liquid Al80Ni20 alloy upon cooling. Our results evidence a huge increase of local icosahedral ordering (ISRO) in the undercooled regime which is more developed around Ni than Al atoms. We show that ISRO has a strong impact on self-diffusion coefficients of both species and is at the origin of their crossover from Arrhenius to non-Arrhenius behavior around a crossover temperature TX = 1000 K, located in the undercooled region. We also clearly identify that this temperature corresponds to the development of dynamic heterogeneities and to the breakdown of the Stokes-Einstein relation. At temperatures below this crossover, we find that the behavior of the diffusion and relaxation dynamics is mostly incompatible with predictions of the mode-coupling theory. Finally, an analysis of the van Hove function indicates that the crossover temperature TX marks the onset of a change in the diffusion mechanism from a normal flow to an activated process with hopping. From these results, the glass-forming ability of the alloy is discussed.

On the role of entropy in determining transport properties in metallic melts.

We performed ab initio molecular dynamics simulations to study the relationship between dynamic and structural properties of liquid Al1-xNix alloys along the T = 1795 K isotherm. Our findings show a strong non-linear dependence of self-diffusion coefficients and the viscosity as a function of composition, in fair agreement with available experimental data. We demonstrate that this non-linear dependence can be related to the detailed description of local structural ordering and more particularly to a pronounced increase of icosahedral short range order (ISRO) in the Ni-rich composition range. We evidence that the composition dependence of local structural ordering is not captured by the Stokes-Einstein relation but can be understood in terms of the partial excess entropy approximated by the two-body contribution.

Structural and physicochemical properties of liquid Al-Zn alloys: a combined study based on molecular dynamics simulations and the quasi-lattice theory.

Ordering phenomena have been investigated in liquid Al-Zn alloys performing molecular dynamics (MD) simulations using "empirical oscillating pair potentials." The local structural order is studied by computing two microscopic functions, namely, the concentration fluctuation function and the Warren-Cowley short-range order parameter. We also study the influence of ordering phenomena on transport properties like diffusivity and viscosity. The MD results are confronted to those determined from measurements and in the framework of the quasi-lattice theory.

Interplay between the structure and dynamics in liquid and undercooled boron: an ab initio molecular dynamics simulation study.

In the present work, the structural and dynamic properties of liquid and undercooled boron are investigated by means of ab initio molecular dynamics simulation. Our results show that both liquid and undercooled states present a well pronounced short-range order (SRO) mainly due to the formation of inverted umbrella structural units. Moreover, we observe the development of a medium-range order (MRO) in the undercooling regime related to the increase of inverted umbrella structural units and of their interconnection as the temperature decreases. We also evidence that this MRO leads to a partial crystallization in the ß-rhombohedral crystal below T = 1900 K. Finally, we discuss the role played by the SRO and MRO in the nearly Arrhenius evolution of the diffusion and the non-Arrhenius temperature dependence of the shear viscosity, in agreement with the experiment.

The hydrogen diffusion in liquid aluminum alloys from ab initio molecular dynamics.

We study the hydrogen diffusion in liquid aluminum alloys through extensive ab initio molecular dynamics simulations. At the microscopic scale, we show that the hydrogen motion is characterized by a broad distribution of spatial jumps that does not correspond to a Brownian motion. To determine the self-diffusion coefficient of hydrogen in liquid aluminum alloys, we use a generalized continuous time random walk model recently developed to describe the hydrogen diffusion in pure aluminum. In particular, we show that the model successfully accounts the effects of alloying elements on the hydrogen diffusion in agreement with experimental features.

Striking role of non-bridging oxygen on glass transition temperature of calcium aluminosilicate glass-formers.

Molecular dynamics simulations are used to study the structural and dynamic properties of calcium aluminosilicate, (CaO-Al2O3)1-x(SiO2)x, glass formers along three joins, namely, R = 1, 1.57, and 3, in which the silica content x can vary from 0 to 1. For all compositions, we determined the glass-transition temperature, the abundances of the non-bridging oxygen, triclusters, and AlO5 structural units, as well as the fragility from the temperature evolution of the α-relaxation times. We clearly evidence the role played by the non-bridging oxygen linked either to Al atoms or Si atoms in the evolution of the glass-transition temperature as well as of the fragility as a function of silica content along the three joins.

Dynamic properties of liquid and undercooled aluminum.

First-principles molecular dynamics simulations of liquid and undercooled aluminum have been performed to study the evolution of dynamic properties across the melting point. Single-atom as well collective dynamic properties are determined and are related to the structural evolution of the liquid phase. The temperature dependence of the self-diffusion coefficient is computed from both the mean square displacement and the velocity autocorrelation function. Self-diffusion coefficients follow an Arrhenius law with the single activation energy of 250 meV, consistent with experimental results for the liquid phase. Moreover, we show that the Arrhenius relation can also be applied to the undercooled state at T = 875 K. Finally, a direct calculation of the shear viscosity from the transverse current correlation function shows that the Stokes-Einstein relation can be applied over the temperature range investigated.

Structural and dynamic properties of calcium aluminosilicate melts: a molecular dynamics study.

The structural and dynamic properties of calcium aluminosilicate (CaO-Al2O3)1-x(SiO2)x melts with low silica content, namely, along the concentration ratio R = 1 are studied by classical molecular dynamics. An empirical potential has been developed here on the basis of our previous ab initio molecular dynamics. The new potential gives a description of the structural as well as the dynamics with a good accuracy. The self-intermediate scattering function and associated α-relaxation times are analyzed within the mode-coupling theory. Our results indicate a decrease of the fragility whose structural origin is a reduction of the number of fivefold coordinated Al atoms and non-bridging oxygen.

Local order and dynamic properties of liquid Au(x)Si(1-x) alloys by molecular dynamics simulations.

Molecular dynamics simulations are performed to examine structural and dynamic properties of liquid Au-Si alloys around the eutectic composition, with interactions described via a modified embedded-atom model suitable for the liquid properties. The local structure as defined by the partial pair-correlation functions, coordination numbers, and partial structure factors is found to display a strong evolution with composition. In addition, a structural study using a three-dimensional pair-analysis technique evidences a strong evolution of the icosahedral short-range order over the range of concentrations, 0 < x(Si) < 0.5. In examining the dynamic properties of these alloys, we show a strong interplay between the structural changes and the evolution of the self-diffusion coefficients as a function of composition.

Signatures of fragile-to-strong transition in a binary metallic glass-forming liquid.

Classical molecular dynamics investigations of the evolution of the microscopic structure and atomic dynamics are found to provide signatures of fragile-to-strong transition in a Cu-Zr bulk metallic glass forming alloy. Present study reveals that (i) the alloy exhibits a non-monotonic decoupling of the self-diffusion coefficient D and the relaxation time τ as observed in case of supercooled water despite the difference in the intermolecular interactions compared to this system, (ii) the temperature dependence of D and τ suggests a crossover from non-Arrhenius to Arrhenius behavior near mode-coupling transition temperature T(C), and (iii) the alloy exhibits a crossover from Stokes-Einstein ((D ~ (τ/T)(-1)) to fractional Stokes-Einstein (D ∝ (τ/T)(-ζ)) with exponent ζ ≈ 0.6. A weak first-order transition, associated with the fragile-to-strong transition, has also been observed in the undercooled region. These findings are in accordance with the growing idea of fragile-to-strong crossover having larger generality than the traditional classification of the glass-forming liquids as fragile and strong.