Contributions to the mixed-alkali effect in molecular dynamics simulations of alkali silicate glasses

Lammert, Heiko; Heuer, Andreas

doi:10.1103/physrevb.72.214202

Cited by 50 publications

(66 citation statements)

References 43 publications

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“…Moreover, it could be shown that the occurrence of large mixed-alkali peaks at small mixing ratios can be understood if the fraction of empty sites is small. This gives independent evidence for the small fraction of empty sites found in theoretical arguments 104 as well as in molecular dynamics simulations 99,100,105,106 (section VII). Despite this progress over the past years, there are still many issues awaiting experimental clarification and theoretical explanation.…”

Section: What Causes the Mixed-alkali Effect?supporting

confidence: 56%

Fundamental questions relating to ion conduction in disordered solids

et al. 2009

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A number of basic scientific questions relating to ion conduction in homogeneously disordered solids are discussed. The questions deal with how to define the mobile ion density, what can be learned from electrode effects, what is the ion transport mechanism, the role of dimensionality, and what are the origins of the mixed-alkali effect, of time-temperature superposition, and of the nearly-constant loss. Answers are suggested to some of these questions, but the main purpose of the paper is to draw attention to the fact that this field of research still presents several fundamental challenges.

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Section: What Causes the Mixed-alkali Effect?supporting

confidence: 56%

Fundamental questions relating to ion conduction in disordered solids

et al. 2009

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“…Previous studies have shown that the local structure and dynamics of silicate glasses could be accurately reproduced using a periodic cell of $10 Å , [35,36] and the compositions that we have investigated in this work are therefore built for a similar cell, according to the experimental densities at room temperature, as reported in Table 1. Since it is commonly difficult for MD simulations to reproduce both experimental density and pressure of glasses, [34,[36][37][38][39][40][41][42] in this study we have decided to reproduce the experimental density. The accumulated average pressure of our models is around 2 GPa, but, according to previous MD studies of silica and phosphosilicate glasses, [43,44] the structural and dynamical properties of glasses at this level of pressure do not significantly change with respect to 1 atm.…”

Section: Simulation Protocolmentioning

confidence: 99%

An Ab Initio Molecular Dynamics Study of Bioactive Phosphate Glasses

2010

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First principles molecular dynamics simulations of ternary phosphate‐based glasses P2O5CaONa2O (PBGs) have been carried out in order to provide an accurate description of the local structure and properties of these important materials for biomedical applications. The structures of PBGs with compositions (P2O5)0.45(CaO)x(Na2O)0.55 − x (x = 0.30, 0.35, and 0.40) were generated using a full ab initio molecular dynamics melt‐and‐quench procedure. The analysis of the structure of the glasses at 300 K shows the prevalence of the metaphosphate Q2 and pyrophosphate Q1 species, whereas the number of Q3 units, which constitute the three‐dimensional phosphate network, significantly decreases with the increase in calcium content in the glass. Calculation of the pair and angular distribution functions suggests that the rigidity of the phosphate tetrahedral glass network increases with the concentration of calcium, an observation which is interpreted in terms of the tendency of Ca2+ to be a stronger coordinator than sodium.

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“…Moreover, it was found that the old idea of ions moving by the vacancy mechanism may well be correct [4], and simulations gave new insight into the mixed-alkali effect [5]. Despite these and other significant advances, important questions remain unanswered.…”

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confidence: 99%

ac Hopping Conduction at Extreme Disorder Takes Place on the Percolating Cluster

2008

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Simulations of the random barrier model show that ac currents at extreme disorder are carried almost entirely by the percolating cluster slightly above threshold; thus contributions from isolated low-activation-energy clusters are negligible. The effective medium approximation in conjunction with the Alexander-Orbach conjecture leads to an excellent analytical fit to the universal ac conductivity with no nontrivial fitting parameters.Recent advances relating to ion conduction in glasses and other disordered solids include the application of multidimensional NMR techniques [1], the introduction of ac nonlinear spectroscopy [2], and elucidations of the high-frequency nearly constant loss [3]. Moreover, it was found that the old idea of ions moving by the vacancy mechanism may well be correct [4], and simulations gave new insight into the mixed-alkali effect [5]. Despite these and other significant advances, important questions remain unanswered. For instance, it is still not understood what role is played by ion interactions for the conductivity [6], or why the random barrier model (RBM) [7,8] represents ac conductivity data so well. The latter question is not answered below, but new simulations and arguments are presented that we believe lead to a full understanding of the physics of the RBM in the extreme disorder limit (low temperature limit).Ac conductivity is often studied also for amorphous semiconductors, electronically or ionically conducting polymers, defective crystals of various kinds, polaronic conductors, etc [7,8]. It is a longstanding observation that all disordered solids have remarkably similar ac conductivities [9]. Universal features include [8]: At low frequencies the conductivity is constant. At higher frequencies it follows an approximate power law with an exponent less than one that increases slightly with increasing frequency. When measured in a fixed frequency range, the exponent converges to one as temperature goes to zero. The ac conductivity is less temperature dependent than the dc conductivity and obeys time-temperature superposition (sometimes referred to as "scaling"). The frequency marking onset of ac conduction, ω m , has the same activation energy as the dc conductivity.These and other observed features are reproduced by the RBM characterized [8,10] by five assumptions: 1) All charge carrier interactions including self-exclusion are ignored; 2) Charge carrier motion takes place on a cubic lattice; 3) All lattice sites have same energy; 4) Only nearest-neighbor jumps are allowed; 5) Jump rates ∝ exp(−E/k B T ) have random activation energies with distribution p(E). In the RBM the ac conductivity σ(ω) relative to σ(0) as a function of a suitably scaled frequency becomes independent of p(E) in the extreme disorder limit, i.e., when the width of p(E) is much larger than k B T [8]. Despite lack of non-trivial free parameters the RBM universal ac conductivity gives a good fit to experiment [8]; more refined models yield results that are close to those of the RBM [11].It is well-known t...

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Contributions to the mixed-alkali effect in molecular dynamics simulations of alkali silicate glasses

Cited by 50 publications

References 43 publications

Fundamental questions relating to ion conduction in disordered solids

Fundamental questions relating to ion conduction in disordered solids

An Ab Initio Molecular Dynamics Study of Bioactive Phosphate Glasses

ac Hopping Conduction at Extreme Disorder Takes Place on the Percolating Cluster

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