EXAFS data analysis for lanthanide sesquioxides

Abstract

The EXAFS spectra of lanthanide sesquioxides (Lu2O3, Sm2O3, La2O3) at the lanthanide LIII-edge are analysed. The complex coordination polyhedron around the lanthanide cation is modelled with the minimum number of coordination shells that still allows the acquisition of coordination numbers and shell distances in agreement with the radial distribution functions from crystallographic data. Theoretical phase shifts and backscattering amplitudes with an amplitude reduction factor, S20= 0.73, are reliable for reproducing the experimental EXAFS data. A model with one Lu–O and two Lu–Lu shells simulates the Lu coordination polyhedron in C-Lu2O3, while for La2O3 the model includes two oxygen shells for simulating the nearest neighbours and one longer distance that averages that of the La–La pairs. The coordination around Sm in Sm2O3 is the most complex and two-shell models are needed to simulate the Sm–O and Sm–Sm absorber–backscatterer pairs. The models obtained are applied in the EXAFS analysis of dispersed Ln2O3/Al2O3 samples, where X-ray diffraction fails to detect the structure adopted by the lanthanide phase. The results show that in an Sm2O3/Al2O3 sample calcined at 800 °C, very small Sm2O3 particles are formed. In an La2O3/Al2O3 sample with low loading the analysis procedure allows the detection of the aluminium atoms that are present with the oxygens around the lanthanum ions, thus suggesting the incipient formation of a bidimensional LaAlO3 phase.