EXAFS analysis of the structural evolution of gel-formed La2O3

Sol-gel-produced La2O3, 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 La2O3 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(CO3)(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 O-17 solid-state NMR on a similar set of samples has some significant implications for the sensitivity to disorder of these important atomic-scale probes.