Space Charge in Ionic Crystals. I. General Approach with Application to NaCl

Kliewer, K. L.; Koehler, J. S.

doi:10.1103/physrev.140.a1226

Cited by 459 publications

(216 citation statements)

References 13 publications

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“…El modelo de Mott-Schottky para la zona de carga espacial establece que la carga segregada al núcleo de la frontera de grano provoca un empobrecimiento de carga en la región del interior del grano adyacente a la frontera (21,22). Como consecuencia la frontera se comporta como una barrera para el transporte iónico perpendicular y provoca un aumento de la resistividad específica de la frontera de grano (r fg ) respecto a la resistividad del volumen (11).…”

Section: Resultados Y Discusiónunclassified

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Frechero¹,

Rocci²,

Schmidt³

et al. 2012

Bol. Soc. Esp. Ceram. Vidr.

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En este trabajo se presentan resultados de medidas de espectroscopia de impedancias realizadas en un bicristal del conductor iónico zirconia estabilizada con itria (YSZ). Utilizando electrodos de tamaño micrométrico se ha podido medir el transporte iónico a través, perpendicularmente, de una única frontera de grano, caracterizando eléctricamente las propiedades de dicha frontera. De este modo se han obtenido los parámetros microscópicos que determinan la distribución de carga en la frontera y por lo tanto el transporte iónico a través de ella, como son la barrera de potencial en la frontera DF = 0.35±0.01 V a 275 ºC, y el espesor de la zona de carga espacial l * = 5±1 Å. Estos valores son significativamente diferentes a los obtenidos anteriormente en muestras cerámicas policristalinas del mismo material, y muestran mejor acuerdo con los valores que predice el modelo de Mott-Schottky para la distribución de carga y el transporte iónico a través de la frontera de grano. Palabras clave: Conductividad. Bordes de grano. Conductores iónicos. Electric characterization of grain boundaries in ionic conductors by impedance spectroscopy measurements in a bicrystalHere we show impedance spectroscopy measurements on a bicrystal of the ionically conducting yttria stabilized zirconia (YSZ). By using micrometer sized electrodes it is possible to measure ionic transport perpendicular to a single grain boundary, and characterize its electrical properties. We are thus able to obtain the microscopic parameters that determine the charge distribution at the grain boundary and the ionic transport through it, as the potential energy barrier DF = 0.35±0.01 V at 275 ºC, and the space charge layer thickness l * = 5±1 Å. These values are significantly different from those previously obtained in polycrystalline ceramic samples of the same material, and show much better agreement with the values predicted by the Mott-Schottky model for the charge distribution and ionic transport through the grain boundary. Kedywords: Conductivity. Grain boundaries. Ionic conductors. INTRODUCCIÓNCuando se reduce alguna de las dimensiones de un material hasta el rango de los nanómetros pueden aparecer comportamientos en sus propiedades físicas que se desvían del comportamiento que presenta el mismo material cuando sus dimensiones son mayores, típicamente por encima de la micra. Esto es debido a que las longitudes características que gobiernan muchos fenómenos físicos son típicamente del orden de unos pocos nanómetros. En los últimos años, debido a la posibilidad de acceder con mayor facilidad a la escala nanométrica, existe un elevado interés por estudiar este tipo de efectos de tamaño sobre las propiedades físicas de los materiales. En particular, en el área de los materiales conductores iónicos, la investigación en este tipo de efectos ha dado lugar a un nuevo campo científico que se ha dado en denominar "nanoiónicos" (1-4). Al disminuir el tamaño de grano en muestras nanocristalinas, o el espesor en películas ultradelgadas, aumenta drásticamente el número y...

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Section: Resultados Y Discusiónunclassified

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Frechero¹,

Rocci²,

Schmidt³

et al. 2012

Bol. Soc. Esp. Ceram. Vidr.

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“…microstructure of interracial defects, especially of grain boundaries. The potential barrier at the grain boundaries-has been explained phenomenologically as a back to back double Schottky barrier [4][5][6]. The origin of the barrier has been attributed to the presence of immobile charge on the grain boundary plane.…”

Section: Introductionmentioning

confidence: 99%

Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO₃

et al. 1999

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The understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which setup a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to resolve the dislocation core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the groin boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.

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“…First, the coherent interface width is large and represents a significant fraction of the period of superlattices [3,4], thus suggesting a very plausible explanation for the observed significant ionic conductivity improvements. Interestingly, this phenomenon is not driven by space charge effects [5,6], the most commonly invoked explanation for longrange deviations from the bulk equilibrium vacancy composition. The composition profile also exhibits composition oscillations, a feature that, to our knowledge, has never been noted in ceramic coherent interfaces.…”

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confidence: 99%

First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

Walle

Ellis

2007

Phys. Rev. Lett.

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Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent f100g interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions. The performance of solid oxide fuel cells [1] is steadily increasing, thanks to the development of novel electrode and electrolyte materials. A recent trend is the use of nanostructured materials to devise electrolytes and electrodes with improved properties [2]. In particular, nanoscale superlattices consisting of layers of the samariumdoped ceria ion conductor with various levels of doping have been proposed as an avenue to improve ionic conductivities of the electrolyte in the low-temperature regime [3,4]. Optimizing this approach as well as understanding the mechanisms underlying the increased ionic conductivity requires a detailed understanding of the interfacial properties in this system.In this Letter, we present ab initio calculations of the equilibrium composition profile (of dopants and vacancies) across an array of coherent f100g interfaces mimicking the experimentally synthesized samarium-doped ceria superlattices. These calculations reveal two surprising findings. First, the coherent interface width is large and represents a significant fraction of the period of superlattices [3,4], thus suggesting a very plausible explanation for the observed significant ionic conductivity improvements. Interestingly, this phenomenon is not driven by space charge effects [5,6], the most commonly invoked explanation for longrange deviations from the bulk equilibrium vacancy composition. The composition profile also exhibits composition oscillations, a feature that, to our knowledge, has never been noted in ceramic coherent interfaces.The determination of the equilibrium f100g interface composition profile in this system demands simulation cell sizes that are beyond the reach of a brute-force ab initio approach: In order to obtain results that are converged with respect to system size, a system consisting of over 70 000 atoms is needed. Moreover, finding the equilibrium profile requires extensive sampling of configuration space that would be prohibitive via ab initio methods alone. To address these issues, we rely on the cluster expansion formalism [7][8][9][10], which represents the energy E of a crystalline alloy with a computationally efficient Hamiltonian taking the form of a polynomial in terms of occupation variables i indicating the type of atom residing on each lattice site i:The unknown coefficients J . . . of this polynomial are called effective cluster interactions (ECI) and are fit to a database of ab initio stru...

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Space Charge in Ionic Crystals. I. General Approach with Application to NaCl

Cited by 459 publications

References 13 publications

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO₃

First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

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Space Charge in Ionic Crystals. I. General Approach with Application to NaCl

Cited by 459 publications

References 13 publications

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Caracterización eléctrica de fronteras de grano en conductores iónicos mediante medidas de espectroscopia de impedancias en un bicristal

Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO3

First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices

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Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO₃