Basic Physics of the Coupling Model: Direct Experimental Evidences

Ngai, K. L.; Rendell, R. W.

doi:10.1021/bk-1997-0676.ch004

Cited by 53 publications

(44 citation statements)

References 39 publications

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“…Interestingly, our results of a concomitant decrease of the activation energy E dc for the dc conductivity and of the value of n, with increasing ordering (see Table 1), can be rationalized in terms of the Coupling Model (CM) (13,17,19,20). The CM starts with the consideration of independent hops of ions to vacant adjacent sites with exponential correlation function, , and relaxation time τ 0 .…”

Section: Electrical Propertiesmentioning

confidence: 70%

“…As this figure shows, the frequency dependence of conductivity at low temperatures may be well described by the so-called Jonscher empirical expression (12), σ'(ω) ∝ ω n , consistent with a power law-type dependence at high frequencies followed by a frequency-independent conductivity plateau associated to the dc conductivity regime, σ dc . This behavior constitutes the main feature of the so-called "universal dielectric response" and has been linked with the existence of cooperative effects in the dynamics of hopping ions (13). The value of the fractional exponent n (0 ≤ n < 1) is determined by the strength of the ionion interactions in the ionic hopping process; i.e.…”

Section: Methodsmentioning

confidence: 99%

“…Different models have been proposed to analyze electrical relaxation data in ionic conductors (13,17,18) which take into account the existence of cooperative effects among mobile ions in the diffusion process. Interestingly, our results of a concomitant decrease of the activation energy E dc for the dc conductivity and of the value of n, with increasing ordering (see Table 1), can be rationalized in terms of the Coupling Model (CM) (13,17,19,20).…”

Section: Electrical Propertiesmentioning

confidence: 99%

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Síntesis y propiedades eléctricas de la solución sólida Gd2-yLayZr2O7 con estructura de tipo pirocloro

Díaz-Guillén¹,

Díaz-Guillén²,

Padmasree³

et al. 2008

Bol. Soc. Esp. Ceram. Vidr.

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Different compositions in the pyrochlore-type Gd 2-y La y Zr 2 O 7 solid solution (0 ≤ y ≤ 1) were prepared at room-temperature by mechanically milling stoichiometric mixtures of the corresponding oxides. Irrespective of their lanthanum content, as-prepared powder samples consist of single-phase anion deficient fluorite materials, although long-range ordering of cations and anion vacancies characteristic of pyrochlores was observed in all cases after firing the samples at 1500°C. Interestingly, activation energy for oxygen migration in the series decreases as La-content increases, from 1.13 eV for Gd 2 Zr 2 O 7 to 0.81 eV for GdLaZr 2 O 7 , whereas ionic conductivity was found to be almost La-content independent, at least for y ≤ 0.8 at T = 500°C and y ≤ 0.4 at T = 800°C. These results are explained in terms of weaker ion-ion interactions in better ordered structures (i.e., as La-content increases) and highlight the importance of structural ordering/disordering in determining the dynamics of mobile oxygen ions. Keywords: Pyrochlores; mechanical milling; zirconates; SOFCs; lanthanum.Síntesis y propiedades eléctricas de la solución sólida Gd 2-y La y Zr 2 O 7 con estructura de tipo pirocloro Partiendo de mezclas estequiométricas de los óxidos correspondientes, se prepararon por molienda mecánica y a temperatura ambiente diferentes composiciones en la solución sólida Gd 2-y La y Zr 2 O 7 (0 ≤ y ≤ 1) con estructura de tipo pirocloro y conductora de iones oxígeno. Independientemente del contenido de lantano, los polvos extraídos del molino presentaron difractogramas similares al de una fluorita no estequiométrica aunque en todos los casos, el tratamiento térmico a 1500°C indujo la aparición del ordenamiento de largo alcance de cationes y vacancias aniónicas característico de pirocloros. La energía de activación para el proceso de migración de iones oxígeno en la serie disminuye a medida que se incrementa el contenido de lantano, desde 1.13 eV de Gd 2 Zr 2 O 7 hasta 0.81 eV de GdLaZr 2 O 7 , mientras que la conductividad resultó ser prácticamente independiente del mismo hasta y ≤ 0.8 para T = 500°C e y ≤ 0.4 para T = 800°C. Estos resultados se explican en términos de una menor interacción entre portadores de carga en las estructuras más ordenadas (a medida que se incrementa el contenido de La) y subraya la influencia del orden/desorden estructural en la dinámica de iones móviles.

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Section: Electrical Propertiesmentioning

confidence: 70%

Section: Methodsmentioning

confidence: 99%

Section: Electrical Propertiesmentioning

confidence: 99%

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Síntesis y propiedades eléctricas de la solución sólida Gd2-yLayZr2O7 con estructura de tipo pirocloro

Díaz-Guillén¹,

Díaz-Guillén²,

Padmasree³

et al. 2008

Bol. Soc. Esp. Ceram. Vidr.

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“…8,20,21 For vibrating ions to come out by thermal activation over an energy barrier E a , the independent relaxation time is given by o (T)ϭ ϱ exp(E a /kT). The reciprocal of ϱ is the vibration angular frequency of the ion.…”

Section: ͑1͒mentioning

confidence: 99%

Correlation between ion hopping conductivity and near constant loss in ionic conductors

2004

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For ionic conductivity relaxation in ionically conducting materials we predict in the framework of the coupling model that the magnitude of the ubiquitous near constant loss correlates with the activation energy E a for independent ion hopping. Using experimental data of a variety of ionic conductors, this correlation has been borne out. The model also explains the observed correlation between the magnitude of the near constant loss and the value of the dc conductivity at room temperature, as well as the temperature dependence for the near constant loss. DOI: 10.1103/PhysRevB.69.134303 PACS number͑s͒: 66.30.Hs Due to their technological importance in state-of-the-art batteries, sensors, and other devices, there is an increasing research activity on ion-conducting materials during last decades. 1,2 The challenge is to clarify the mechanism that limits the mobility of ions and consequently break new grounds for practical applications. Therefore, efforts have been focused on the dynamics of ionic hopping transport at low frequencies and near room temperature. 3,4 Only recently, the ac conductivity of ionic conductors at high frequencies and low temperatures has attracted the attention of workers in the field. 5-8 A ubiquitous nearly linear frequency dependent term Ј( )ϷA o 1Ϫ␣ with ␣ almost zero and o the permittivity of a vacuum, becomes the dominant contribution in the ac conductivity at sufficiently high frequencies or low enough temperatures. This term is equivalent to a nearly frequency independent dielectric loss, ͑1͒and naturally this contribution is known as the near constant loss ͑NCL͒. 9 The properties of the NCL are lesser known compared with the dc conductivity at room temperature in ionic conductors, and, at present there is an ongoing debate on the origin of the NCL. 8,10,11 On the other hand, it was pointed out that superionic conductors, characterized by their exceptional high dc conductivity at room temperature, show also the highest values of the near constant loss at low temperatures. 9 We show in this work that there exists a strong correlation, for a variety of ionic conductors, between the magnitude of the NCL observed at low temperatures and the dc conductivity and its activation energy at roomtemperature. Therefore, understanding the origin of the NCL also may help to improve the understanding of the dynamics of ionic conductivity and the factors limiting the roomtemperature dc conductivity in ionic conductors. 12 The NCL appears at higher frequencies than the ion hopping ac conductivity, hop Ј ( ). The latter is assumed by some workers to be represented by the Jonscher expression 13where n J is a fractional exponent, 0 is the dc conductivity, and p a characteristic relaxation frequency. Both 0 and p are found to be thermally activated with about the same activation energy E . Alternatively, in terms of the electric modulus, 14 the ion hopping ac conductivity is also well described by the one-sided Fourier transform,of the time derivative of a stretched exponential function ⌽(t)ϭexp͓Ϫ(t/...

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“…Even when vestiges of many-body dynamics are observed, their effect is only rationalized ad hoc but no prediction is made concerning the direct connection of the many-body dynamics to τ α . An exception to this usual theoretical approach is the coupling model (CM) [5][6][7][8], whose defining prediction,…”

Section: Introductionmentioning

confidence: 99%

Guides to solving the glass transition problem

Ngai¹,

Prevosto²,

Capaccioli³

et al. 2008

J. Phys.: Condens. Matter

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Relaxation in glass-forming substances is necessarily a many-body problem because of intermolecular interactions and constraints. Results from molecular dynamics simulations and experiments are used to reveal the critical elements and general effects originating from many-body relaxation, but not dealt with in conventional theories of the glass transition. Although many-body relaxation is still an unsolved problem in statistical mechanics, these critical elements and general effects will serve as guides to the construction of a satisfactory theory of the glass transition. This effort is aided by concepts drawn from the coupling model, whose predictions have been shown to be consistent with experimental facts.

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Basic Physics of the Coupling Model: Direct Experimental Evidences

Cited by 53 publications

References 39 publications

Síntesis y propiedades eléctricas de la solución sólida Gd<sub>2-y</sub>La<sub>y</sub>Zr<sub>2</sub>O<sub>7</sub> con estructura de tipo pirocloro

Síntesis y propiedades eléctricas de la solución sólida Gd<sub>2-y</sub>La<sub>y</sub>Zr<sub>2</sub>O<sub>7</sub> con estructura de tipo pirocloro

Correlation between ion hopping conductivity and near constant loss in ionic conductors

Guides to solving the glass transition problem

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