A model for the onset of fast-ion conduction in fluorites

Allnatt, A. R.; Chadwick, Alan V.; Jacobs, P. W. M.

doi:10.1098/rspa.1987.0045

Cited by 23 publications

(6 citation statements)

References 48 publications

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“…In terms of the graph, these numbers are equal to the numbers of upward and Now we want to return to the idea that is sometimes put forward, that diffusion is due almost entirely to vacancies, while the interstitials are immobilized on additional sites (se e.g. Moscinski and Jacobs 1985, Allnatt et al 1987). Can this idea be reconciled with the form of the hopping diagram shown in Figure 26?…”

Section: We Now Want To Illustrate the Kind Of Results Produced By Thementioning

confidence: 92%

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Dynamical Simulations of Superionic Conductors

Gillan

1989

Series on Directions in Condensed Matter Physics

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Superionic conductors are ionic solids in which one ion species is disordered andhighly mobile. Molecular dynamics (MD) simulation gives a powerful way of studying models of superionic conductors. We explain the MD technique and describe how it is applied to ionic systems. We than show how simulation of fluorite superionic conductors has successfully reproduced a range of experimental measurements on quantities including the diffusion coefficient, the spatial distribution of disordered ions, the van Hove self-function and the dynamical structure factors. The simulations have been used to provide a unified interpretation of the observations. This shows that superionic conduction in fluorites occurs by the motion of vacancy and interstitial defects, both of which jump between regular lattice sites, the coherent quasielastic peak recently observed by neutron scattering is shown to arise from the motion of these defects. Ionic Solids at High Temperatures Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 09/30/15. For personal use only. 170 IntroductionAll crystalline materials show some degree of disorder at high temperatures. This disorder is of two kinds: firstly, there are vibrational displacements of the ions from their regular sites, and secondly, there are point defects. The presence of point defects -vacancies and interstitials -is responsible for the diffusion of ions that is observed in all ionic crystals. Vibrational motions are usually treated in the harmonic approximation, which assumes that the displacements are small. As one goes to higher temperatures, this approximation becomes less and less valid and anharmonic effects become important: the lattice parameter depends on temperature, the constant-pressure and constant-volume specific heats differ, and the vibrational modes become damped. A more sophisticated theoretical description is then called for -an example is the self-consistent phonon approximation described by Ball elsewhere in this book. The point-defect disorder is also easiest to discuss when the temperature is not too high. One can then use energy-minimization techniques to determine the energies of formation and migration of the defects, and harmonic theory to calculate the associated entropies. These highly successful methods are discussed in the article by Harding. Again, at very high temperatures one encounters difficulties, especially if the defect concentration becomes high and the defects begin to interact with each other.

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Section: We Now Want To Illustrate the Kind Of Results Produced By Thementioning

confidence: 92%

“…It has often been assumed in the past (see e.g. Mo£ctiiski and Jacobs 1985, Allnatt et al 1987) that since diffusion occurs entirely by jumps between the anion regular sites, a vacancy mechanism must be dominant. We want to stress here the reason why this assumption is unjustified.…”

Section: Discussionmentioning

confidence: 99%

Dynamical Simulations of Superionic Conductors

Gillan

1989

Series on Directions in Condensed Matter Physics

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“…At temperatures close to, but below, their melting points cubic fluorite structured materials are known to undergo a transition to the "superionic" regime, characterized by a dramatic increase in ionic conductivity [42][43][44] while electronic conductivity remains low. This transition is associated with a large dynamic disorder of the anion sublattice, where transport of anions occurs via a hopping mechanism, 42,[45][46][47][48] though the details are still not completely understood. 44 This mechanism of conduction may play a role in ceria at elevated temperatures; superionic conductivity has been observed in ultrathin Gd-doped ceria 49 and Sm-doped ceria nanocomposites 50 at relatively low temperatures (under 900 K), but in these cases was attributed to grain-boundary and size effects.…”

Section: Introductionmentioning

confidence: 99%

Dynamical response and instability in ceria under lattice expansion

et al. 2013

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We present results of density functional theory calculations on the phonon dispersion and elastic constants of bulk ceria (CeO 2) as a function of positive and negative isotropic strain, which could be induced thermally or by cationic doping. We find that, as the lattice is expanded, there is a significant softening of the B 1u mode at the X point. This mode consists of motions of oxygens in the [001] direction. At a strain of 1.6%, corresponding to a temperature of 1600 K, the B 1u and E u modes at the X point cross, with an associated high, narrow peak in the phonon density of states appearing. We infer that this crossing indicates a coupling of the modes, leading to a transition to a superionic phase, where conductivity occurs in the [001] direction, mediated by anion interstitial site occupation. As the lattice is expanded further, the B 1u mode continues to soften, becoming imaginary at a strain of 3.4%, corresponding to a temperature of 2500 K. Following the imaginary mode would result in a cubic to tetragonal phase transition, similar to those known to occur with reducing temperature in zirconia (ZrO 2) and hafnia (HfO 2). Our calculated elastic constants, however, indicate that the structure remains mechanically stable, even at this level of expansion. As confirmed by our semiclassical free energy calculations, the cubic phase of ceria remains the most stable, while the imaginary mode indicates a change to a thermally disordered cubic phase, with the majority of disorder occurring on the anion sublattice. Our results explain the high temperature ionic conductivity in ceria and other fluorite-structured materials in terms of the intrinsic lattice dynamics, and give insight to the stability and anionic disorder at elevated temperatures.

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“…While several quasi-static defect cluster models have been proposed before [38], none of them has the ability to identify realistic ionic conduction or diffusive pathways. The goal of this paper has been to investigate the dynamics of Li ions in the superionic state using atomistic simulations and propose a mechanism that can potentially pinpoint the origin of enhanced diffusivity of Li ions in Li 2 O.…”

Section: Discussionmentioning

confidence: 99%

Ion Hopping and Constrained Li Diffusion Pathways in the Superionic State of Antifluorite Li2O

Annamareddy

Eapen

2017

Entropy

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Li 2 O belongs to the family of antifluorites that show superionic behavior at high temperatures. While some of the superionic characteristics of Li 2 O are well-known, the mechanistic details of ionic conduction processes are somewhat nebulous. In this work, we first establish an onset of superionic conduction that is emblematic of a gradual disordering process among the Li ions at a characteristic temperature T α (~1000 K) using reported neutron diffraction data and atomistic simulations. In the superionic state, the Li ions are observed to portray dynamic disorder by hopping between the tetrahedral lattice sites. We then show that string-like ionic diffusion pathways are established among the Li ions in the superionic state. The diffusivity of these dynamical string-like structures, which have a finite lifetime, shows a remarkable correlation to the bulk diffusivity of the system.

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A model for the onset of fast-ion conduction in fluorites

Cited by 23 publications

References 48 publications

Dynamical Simulations of Superionic Conductors

Dynamical Simulations of Superionic Conductors

Dynamical response and instability in ceria under lattice expansion

Ion Hopping and Constrained Li Diffusion Pathways in the Superionic State of Antifluorite Li2O

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