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We acknowledge financial support by Spanish MICINN through grants MAT2011-27470-C01 and Consolider Ingenio 2010 - CSD2009-00013 (Imagine), by CAM through grant S2009/MAT-1756 (Phama) and by the ERC starting Investigator Award, grant #239739 STEMOX. RS wishes to acknowledge the MICINN (Spain) for granting a Ramon y Cajal fellowship. MAF is Research Fellow of CONICET (Argentine). Financial support of CONICET is gratefully acknowledged. OJD acknowledges a postdoctoral fellowship from JCCM. JS acknowledges a Juan de la Cierva fellowship from MICINN (Spain). The authors thank Dr. Hugo Schlich from Mateck GmbH for providing information about the process of bicrystals production, Masashi Watanabe for the Digital Micrograph PCA plug-in and J. Luck for help with specimen preparation, and A. de Andrés for helpful discussion and assistance on experimental matters. Electron microscopy observations at ORNL (SJP, MV) were sponsored by the Materials Sciences and Engineering Division of the U.S. Department of Energy (DOE) and and through the Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, DOE-BES. ESM measurements (SVK, SJ, AK) were performed at the Center for Nanophase Materials Sciences and supported by the Division of Scientific User Facilities of the U.S. Department of Energy (DOE)., The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary., Ministerio de Ciencia e Innovación (MICINN), Comunidad de Madrid, Consolider Ingenio 2010, ERC starting Investigator Award, CONICET, JCCM, Materials Sciences and Engineering Division of the U.S. Department of Energy (DOE), Center for Nanophase Materials Sciences (CNMS), Scientific User Facilities Division, DOE-BES, Division of Scientific User Facilities of the U.S. Department of Energy (DOE), Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

© 2008 The American Physical Society. This work has been carried out with the financial support of Mexican Conacyt Grant No. SEP-2003-C02-44075 and Spanish MCYT Contracts No. MAT2005-06024-C02 and No. MAT2008-06517-C02. M.R.D.G. thanks Conacyt for financial support. K.L. Ngai was supported in part by ONR under Program Element and Project 61153N., In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, E_(dc), in highly disordered pyrochlore-type ionic conductors A_(2)B_(2)O_(7). Twenty six compositions with the general formula, Ln_(2)Zr_(2−y)Ti_(y)O_(7), Ln_(1.7)Mg_(0.3)Zr_(2)O_(7) (Ln=Y, Dy, and Gd), and Gd_(2−y)La_(y)Zr_(2)O_(7), were prepared by mechanical milling, and their electrical properties were measured by using impedance spectroscopy as a function of frequency and temperature. By using the coupling model we also examine the effect of cation radii R_(A) and R_(B) on the microscopic potential-energy barrier, E_(a), which oxygen ions encounter when hopping into neighboring vacant sites. We find that, for a fixed B-site-cation radius R_(B), both activation energies decrease with increasing A-site-cation size, R_(A), as a consequence of the increase in the unit-cell volume. In contrast, for a given R_(A) size, the E_(dc) of the Ln_(2)Zr_(2−y)Ti_(y)O_(7) series increases when the average R_(B) size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with R_(B)., Mexican Conacyt, Spanish MCYT, Conacyt ONR under Program Element, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

© 2010 The American Physical Society. This work has been supported by Mexican CONACYT Grant No. SEP-2003-C02-44075, by Spanish MICINN under Projects No. MAT2008-6517-C02 and CONSOLIDER INGENIO 2010 No. CSD2009-00013 IMAGINE, and by CAM under PHAMA Grant No. S2009/MAT-1756. M.R.D.-G. thanks CONACYT for a grant to stay at Universidad Complutense., We report on ac conductivity measurements of oxide ion conductors with composition Gd_(2)(Zr_(y)Ti_(1−y)_(2)O_(7), at temperatures between 170 and 500 K and in the frequency range 1 Hz–3MHz, and show that a crossover from a sublinear power law to a linear frequency dependence (or nearly constant loss behavior) in the ac conductivity can be clearly observed in a wide temperature range. This crossover is found to be thermally activated, and its activation energy ENCL to be much lower than the activation energy Edc for the dc conductivity. We also found that the values of ENCL are almost independent of composition, and therefore of the concentration of mobile oxygen vacancies, unlike those of Edc. Moreover, for each composition, the values of ENCL=0.67 +/- 0.04 are very similar to those estimated for the energy barrier for the ions to leave their cages, Ea=0.69 +/- 0.05. These results support that the nearly constant loss behavior, ubiquitous in ionic conductors, is originated from caged ion dynamics., Comunidad de Madrid, Ministerio de Economia y Competitividad (MINECO), Mexican CONACYT, CONSOLIDER INGENIO 2010, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

© Sociedad Española de Cerámica y Vidrio. This work has been carried out with the financial support of Mexican Conacyt (Grant SEP-2003-C02-44075) and Spanish MCYT (MAT2004-3070-C05)., 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., Partiendo de mezclas estequiométricas de los óxidos correspondientes, se prepararon por molienda mecánica y a temperatura ambiente diferentes composiciones en la solución sólida 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., Mexican Conacyt, Spanish MCYT, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub