EXAFS analysis of the structural evolution of gel-formed La2O3

Abstract

Sol–gel-produced La ₂ O ₃ , derived from the hydrolysis and subsequent calcination of lanthanum isopropoxide, has been investigated by EXAFS of the La L _III edge. The evolution of the local structure through a series of amorphous intermediate states has been modelled by three shells, two lanthanum–oxygen and the other lanthanum–lanthanum. For comparison, well characterised bulk La(OH) ₃ , LaO(OH) and La ₂ O ₃ were examined using EXAFS. The sol–gel-prepared oxide was more disordered and had significantly smaller particles than the oxide prepared by calcination of La ₂ (CO ₃ ) ₃ after the same heat treatments. For the gels in the intermediate states, well defined La–O shells are seen but the La–La shells are characterised by large Debye–Waller factors. Comparison of these results with previous ¹⁷ O solid-state NMR on a similar set of samples has some significant implications for the sensitivity to disorder of these important atomic-scale probes.