Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites

The standard and anomalous crystal-field spectra of Eu3+

Synthesis and photoluminescence properties of Eu3+-doped AZrO3 (A = Ca, Sr, Ba) perovskite

Preparation and luminescence properties of Eu3+-doped MSnO3 (M = Ca, Sr and Ba) perovskite materials

Coherent multidimensional vibrational spectroscopy (CMDVS) is an emerging field that offers new oportunities to probe specifically intramolecular and intermolecular interactions in complex samples with the improved resolution and selectivity expected of a multidimensional method. In particular, it provides structural and dynamical information about correlations between modes. It provides this information on the picosecond time scales characteristic of vibrational dephasing. CMDVS is based on the perturbations that are induced in a mode when other coupled modes are excited. It is the vibrational analogue to multidimensional nucler magnetic resonance (NMR). In this review, we present the background that led to the development of CMDVS, a brief summary of the different methods of non-linear laser spectroscopy and how they can achieve CMDVS, the relationships between CMDVS and multidimensional NMR, and the theory necessary to understand the experimental results. The experimental approaches reviewed fall into ...

Coherent multidimensional vibrational spectroscopy

Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling betwee...

Molecular, multiresonant coherent four-wave mixing spectroscopy

A family of nonlinear four-wave mixing techniques that are capable of site-selective organic spectroscopy are presented. Three lasers are used in the methods in order to achieve fully resonant mixing. Three lasers are shown to provide better sensitivity, selectivity, and versatility in the study of ground and excited electronic state vibrational spectroscopy. New approaches become possible in the establishment of resonances that translate the output signal from the normal Stokes or anti-Stokes side of the lasers to intermediate positions that are free of fluorescence interference. These new methods are divided into Multiply Enhanced Parametric Spectroscopy (MEPS) and Multiply Enhanced Nonparametric Spectroscopy (MENS), depending upon the spectroscopic characteristics for site-selective applications. The characteristics of MEPS and MENS are found to be quite different and depend upon the number and separation of the sites, the power of the lasers, the relative shifts of the levels, and the correlation effects in the inhomogeneous broadening. The feasibility of MENS and the site-selective capability of both CARS and MENS is demonstrated experimentally with the use of the pentacene: p-terphenyl system as a model.

Site-Selective Nonlinear Four-Wave Mixing by Multiply Enhanced Nonparametric and Parametric Spectroscopy

Identification des spectres des sites dominants qu'on attribue a la substitution par Eu 3+ sur des sites Sr avec une compensation de charge. Les spectres des echantillons non stœchiometriques montrent que la non-stœchiometrie est accommodee par la formation de phases de Ruddlesden-Popper. Observation de centres supplementaires lorsque la concentration en dopant augmente

Site-selective spectroscopy of defect chemistry in SrTiO3, Sr2TiO4, and Sr3Ti2O7.

On tente de savoir si la substitution du dopant est aleatoire ou si des amas preferentiels de plusieurs ions Pr 3+ se forment avec les ions interstitiels de fluorure a compensation de charge. Aux tres basses concentrations dopant, un site Pr 3+ unique compense localement domine le spectre. Il correspond aux dipoles simples qu'on observe par courant thermique ionique et relaxation dielectrique. Pour une concentration de 1 mol%, on observe 3 centres associes a des amas preferentiels. Ces observations prouvent que la substitution n'est pas aleatoire

Steven H. Lee

Papers

Molecular, multiresonant coherent four-wave mixing spectroscopy

Site-Selective Nonlinear Four-Wave Mixing by Multiply Enhanced Nonparametric and Parametric Spectroscopy

Site-selective spectroscopy of defect chemistry in SrTiO3, Sr2TiO4, and Sr3Ti2O7.

Clustering and percolation of defects in Pr3+:SrF2 using site-selective spectroscopy.