Ba
0,77Ca
0,23TiO
3:Nd
3+POR ESPECTROSCOPIA DE ABSORC ¸ ˜ AO RAIOS-X.
Moraes, A. P. A.1, SOUZA FILHO, A. G.2, Azevedo, G. de M.3, Antonelli, E.4, and MPeko, J.-C.4
1 Universidade Federal do Cear´a - Fortaleza CE Brazil
2 Universidade Federal do Cear´a - FORTALEZA CE Brazil
3 Laborat´orio Nacional de Luz S´ıncrotron - Campinas SP Brazil
4 Universidade de S˜ao Paulo - S˜ao Carlos - S˜ao Carlos SP Brazil
Neste trabalho estudamos a estrutura atˆomica local do Ba0,77Ca0,23TiO3 puro e dopado com Nd3+ via Espectroscopia de Absor¸c˜ao de Raios-X nas bordas K do Titˆanio (Ti) e C´alcio (Ca). A regi˜ao da pr´e-borda do espectro de absor¸c˜ao da borda K do Ti apresenta trˆes bordas (A,B,C) bem definidas. A borda “A” corresponde a transi¸c˜ao quadrupolar 1s-3d no ´atomo de Titˆanio. A borda “B” tamb´em possui a componente quadrupolar 1s-3d e uma componente dipolar para o estado p induzida pela hibridiza¸c˜ao dos estados 3d do Ti com os estados p dos oxigˆenios vizinhos ao Titˆanio. A intensidade do pico B depende fortemente da natureza da Perovskita.
A borda “C” ´e devido aos vizinhos do ´atomo de Ti e tem um car´ater dipolar.
Na borda K do C´alcio (Ca) existe uma pr´e-borda similar `a encontrada em metais de transi¸c˜oes 3d. A borda “A” ´e normalmente atribu´ıda `a transi¸c˜oes eletrˆonicas a partir dos estados 1s do Ca ligados aos orbitais moleculares 2p do oxigˆenio ou 3d do Ca e suas origens podem estar relacionadas ao rearranjo dos ´atomos de oxigˆenio vizinhos aos ´ıons do Ca. A borda B, C e D s˜ao originadas principalmente das transi¸c˜oes 1s-4p do Ca de acordo com a regra de sele¸c˜ao do momento de dipolo.
C´alculos de primeiros princ´ıpios foram realizados usando uma super-c´elula (2x2x2) Ba0,77Ca0,23TiO3 usando o formalismo LDA implementado no c´odigo CASTEP e base do tipo DZP. Os resultados te´oricos s˜ao discutidos comparando com os resultados experimentais na tentativa de identificar as transi¸c˜oes observadas em cada borda.Os resultados obtidos por EXAFS mostram que o ´ıon Nd3+ ocupa o s´ıtio A quando incorporado na estrutura do BCT23 puro.
Acknowledgements: Este trabalho foi financiado pelo CNPQ e LNLS.
Structural characterization of La
0.80Sr
0.30Mn
0.90Cr
0.10O
3Cavalcante, I. P.1, Corrˆea, H. P. S.1, Dorot´eia F. Bozano ou D. F. Bozano2, Orlando, M. T. D.3, and Martinez, L. G.4
1 Universidade Federal do Mato Grosso do Sul - Campo Grande MS Brazil
2 Universidade Federal de Mato Grosso - Cuiab´a MT Brazil
3 Universidade Federal do Esp´ırito Santo - Vit´oria ES Brazil
4 Instituto de Pesquisas Energ´eticas e Nucleares - S˜ao Paulo SP Brazil
In recent years, the development of conductive ceramics has been intensified not only due to demands in several areas, especially for their application in Solid Oxide Fuel Cells (SOFCs). The development of low internal resistance SOFC allows the increase of energy conversion efficiency as well as cost reduction. Nowadays the re- search efforts are concentrated mainly in the search for materials compatible to high temperature (approx. 800◦C) operation, aiming to the elimination or reduction of thermal stress and losses in physical-chemical properties due to reactions among component materials, with consequent loss of performance. The lanthanum man- ganite (electrode material) and the lanthanum chromite (interconnector material) are constituent parts of SOFCs. In the present work the lanthanum manganite and chromite are studied in a solid solution, obtained via solid state reaction, present- ing the composition La0.80Sr0.30Mn0.90Cr0.10O3. In its structural characterization, preliminary analyses performed with copper radiation in conventional diffractome- ter showed a non efficient contrast between Mn and Cr. By means of synchrotron light we could make use of anomalous scattering, by collecting data close to the absorption edge of Cr (5.9625 keV). The measurement at the energy of 5.9625 keV produced a better contrast between the scattering factors of Cr and Mn atoms.
The measurements were carried out at D12A XRD1 LNLS beamline.
Acknowledgements: The authors thank the LNLS for the use of its facilities and for technical support.
Electronic structure, stability and surface tension of ZrO
2nanoparticles
Casali, R.A.1, Ponce, C.A.1, and Caravaca, M.A.1 Universidad Nacional del Nordeste - Corrientes Argentina
Based on previous experience in the characterization of group IV-B oxides, in dif- ferent phases, simulated with the DFT based SIESTA code, small clusters and nanoparticles ofZrO2, are studied up to a maximum size of 2.4 nm. The particle total energy is minimized by allowing the full relaxation of atomic coordinates.
Nanoparticles with tetragonal symmetry and with different shapes and surface terminations, are here studied. Density of electronic states are characterized and correlated with structural order/disorder of the nanoparticle. The relation between size of the crystalline core, surface tension is attempted to found as a function of the size and shape of the particle, and surface termination. In order to mimic real nanoparticles, the change on the electronic structure and atomic coordinates are also studied upon doping with an oxygen vacancy. The present results conduct to the determination of particle internal pressure, which is though to correlate with phase transitions under extreme conditions, like high pressures.
Acknowledgements: This work was supported by Facultad de Ciencias Exactas and Fac- ultad de Ingenieria, SECYT-UNNE and CONICET (Argentina) and by the Laboratorio Nacional Luz Sincrotron (Campinas, Brasil)