"XAFS study of Zr local environment in amorphous precursors of La2Zr2O7 ceramics prepared by a nitrate-modified alkoxide synthesis route
published: Feb. 12, 2008, recorded: October 2007, views: 6256
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We prepared La2Zr2O7 by the nitrate-modified alkoxide-based sol-gel synthesis route. The dehydrated lanthanum nitrate was mixed with zirconium n-butoxide in 2-methoxyethanol and refluxed for 3 h. The obtained liquid was slowly dried. At 150oC, the sample self-ignited. According to XRD, the powder was amorphous upon heating up to 700oC and crystallization took place at 800oC. The powder was composed of friable agglomerates of about 70 nm sized nano-particles. By EXAFS analysis we investigated the Zr and La local environment upon transition from the sol to the amorphous powder in order to learn about the distribution of the constituent metal atoms at the sub-nanometer level. We observed that the Zr EXAFS spectra of the sol and the precursor powder dried at 150 oC and heated at 500 oC were similar, indicating that the local environment of Zr atoms was not affected during the liquid-amorphous solid transition, Zr – O – Zr links were formed already in the sol and preserved even in the powder heated at 500 oC. No La – O – Zr links in the sol or in the amorphous powder were observed. The La EXAFS spectrum of the LZ sol is similar to the spectrum of dehydrated lanthanum nitrate (LN sol), while in the spectra of dried and heated powder, vanishing of the second peak was noticed. No La-La or La-Zr corelation could be established. Since no link, between La and Zr species, was confirmed in the early stages of the synthesis, the reaction between the species proceeded as a solid-state reaction when long range diffusion became available leading to the pure pyrochlore phase. The proposed synthesis route, allowed a very good mixing of the species at the nanometre level.
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