Year: 2011
Type: article
Abstract: Sessions C584In the present work, we investigate the influence of crystallite size on the crystal structure, local order and Co oxidation state of LSC nanopowders synthesized by two different chemical methods: citratecomplexation method (C) and stoichiometric nitrate-glycine solutioncombustion process (G) [2].G-process yields porous and deagglomerated powders consisted of nanosized crystallites (average crystallite size, D = 25 nm) that exhibit single perovskite phase after calcination at 900ºC 5 hours.Citrate complexation method produce foamy powders, but the perovskite phase is achieved at 1100ºC.This promotes crystallite growing up to sub-micron sizes (D>200 nm).XRD patterns showed the retention in metastable form at room temperature of the high temperature cubic phase for the nanopowders synthezised by G-process, while powders with bigger crystallite size exhibit the common rhombohedral phase.X-ray absorption spectra (XAS) of the Co K-edge (7709 eV) were recorded at the D04B-XAFS1 beamline of LNLS, Brazil.Data were collected at room temperature in air.WinXas and fdmnes codes were used in the analysis.Changes seen in the 2 nd derivative of the X-ray absorption near edge spectra (XANES) in the pre-edge region, may be ascribable to the differences observed in the crystal structure and also to oxygen vacancies, as shown by simulations using fdmnes code.Also, experimental XANES may be explained in terms of oxygen vacancies.Remarkably, Co(IV) was not needed in these analyses.Some authors claimed Co(IV) appears when Sr substitutes La in LSC [3].This idea goes against the goal of introduce Sr(II) in the A-site to generate oxygen vacancies, based on unit cell neutrality.Accordingly, Fourier transform of EXAFS signal shows an increase of the concentration of oxygen vacancies in the nanopowdered samples.In view of the results of this investigation, the enhanced electrochemical performance of nanostructured LSC cathode may be ascribed to a larger concentration of oxygen vacancies, induced by the small crystallite sizes, in the order of 25 nm.
Authors: A. Chatkon, K. J. Haller, D. C. Crans
Language: English
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2317690272