Electrical conductivity measurements of the fast ionic conductor Li 0.18 La 0.61 TiO 3 have been conducted at temperatures ranging from 8 to 300 K and frequencies between 20 Hz and 5 MHz. A detailed analysis of the ac conductivity shows the existence of a crossover between two different regimes. At high temperatures and/or low frequencies correlated ion hopping is responsible for a power-law frequency dependent and thermally activated ac conductivity. On the other hand, at sufficiently low temperatures and/or high frequencies, the ions do not have enough thermal energy or time to hop between neighboring sites, and remain caged. The ac conductivity is then characterized by a linear frequency dependence ͑i.e., the equivalent of a nearly constant loss͒ and by a weak exponential temperature dependence of the form exp(T/T 0 ). A crossover between the two regimes is found, which is thermally activated with an activation energy Eϭ0.17 eV, significantly lower than that observed for the dc conductivity, E ϭ0.4 eV. From this result, it is shown that the so-called ''augmented Jonscher expression'' fails to describe the ac conductivity in the whole frequency and temperature ranges. All these findings suggest that the nearly constant loss originates from electrical loss occurring during the time regime while the ion is still confined in the potential-energy minimum. Further, it is proposed that the loss mechanism involves some type of process where the potential-energy minimum relaxes in time on a time scale much shorter than the ionic hopping time scale. At longer times, as soon as the ion has significant probability of being thermally activated out of the potential well, the nearly constant loss terminates and correlated ion hopping becomes the only contribution to the ac conductivity.
Electrical relaxation data of crystalline yttria-stabilized zirconia are used to analyze the permittivity change observed in the spectra of the real part of the permittivity in ionic conducting materials. It is found that this permittivity change is independent of both temperature and mobile-ion concentration, and it is determined solely by the degree of interaction among ions in the relaxation process. This finding is at odds with an expression for the permittivity change in the framework of a proposed universal ac conductivity scaling law for glassy ionic conductors. On the other hand, not only the total permitivity change, but also the particular frequency dependence of the permittivity spectra is found to be consistent with the analysis of electrical relaxation in terms of the electric modulus. The results of this work give further support to the use of the electric modulus in describing electrical relaxation in ionic conductors.
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...
Presentamos medidas de la conductividad eléctrica del sistema BIFEVOX Bi 4 V 2-x Fe x O 11-y (0≤x≤0.9; 0≤y≤1), en el que se realiza la sustitución de iones V (IV) por Fe (III) de forma sistemática. La conductividad muestra un comportamiento potencial con la frecuencia, descrito por σ * (ω)=σ dc [1+(jω/ω p ) n ], y conocido como respuesta dieléctrica universal. Análogamente, el módulo eléctrico presenta picos asimétricos, cuya función de relajación en el dominio del tiempo puede describirse mediante exponenciales "estiradas" de la forma φ(t)=exp(-(t/τ σ ) β ). β da cuenta del grado de correlación del transporte iónico, siendo su valor, β=0.56±0.03, casi independiente de la temperatura y del contenido en Fe. Con el aumento en el contenido de Fe, la conductividad disminuye exponencialmente y la energía de activación del proceso de conducción aumenta de 0.20 a 0.97 eV. Estos resultados se discuten en términos de la ordenación de vacantes oxígeno al dopar con Fe (III). Palabras clave: conductor iónico, difusión de oxigeno, espectroscopía de impedancias, correlación iónica. Electrical conductivity and oxygen diffusion in Bifevox.We present electrical conductivity measurements of BIFEVOX Bi 4 V 2-x Fe x O 11-y (0≤x≤0.9; 0≤y≤1), in which V (IV) ions have been systematically substituted by Fe (III) ions. Conductivity shows a power law frequency dependence described by the form σ * (ω)=σ dc [1+(jω/ω p ) n ], known as universal dynamic response. Conversely, the electric modulus shows asymmetric peaks, characterized by stretched exponentials relaxation functions in time domain of the form φ(t)=exp(-(t/τ σ ) β ). β is determined by the degree of correlation in the ionic motion. It´s value, β=0.56±0.03, is almost independent of temperature and iron content. Increasing Fe content leads to an exponential decrease of the conductivity and to an increase of the activation energy of the conduction process from 0.20 to 0.97 eV. These results are discussed in terms of oxygen vacancy ordering upon Fe (III) substitution.
We examine the interplay between ferromagnetism and superconductivity in bilayer and trilayer heterostructures based on Co, YBa 2 Cu 3 O 7−␦ ͑YBCO͒, and La 0.7 Ca 0.3 MnO 3 ͑LCMO͒ thin films grown on SrTiO 3 substrates with typical thicknesses of 10-15 nm. We have measured magnetoresistance below the resistivesuperconducting onset of the YBCO. Naturally oxidized antiferromagnetic CoO top layer films give rise to pronounced exchange bias, modifying the coercive field of the Co by several hundred Oe. This allows separating effects at coercivity, such as stray fields, from those of parallel vs. antiparallel magnetic alignment between top and bottom ferromagnetic layers. In bilayers of Co/YBCO and of LCMO/YBCO, we observe a small magnetoresistance peak centered at the coercive field of the ferromagnetic layer of at most 20%, which we attribute to the effect of stray fields generated in the domain state of the ferromagnet. In the case of the CoO/ Co/ YBCO/ LCMO/ SrTiO 3 trilayer, aside from the peaks at coercivity, we observe a well-defined plateau of the magnetoresistance extending between the coercive fields of the LCMO and Co, with a width that is modified by the exchange-biased Co layer. Reactivity between Co and YBCO at the interface gives rise to a progressive deterioration in the superconducting transition temperature. Aged samples display magnetoresistance peaks at the coercive fields of the Co and LCMO characteristic of stray fields without the magnetoresistance plateau between them.
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