FLUORITE-TYPE SOLID SOLUTIONS IN THE SYSTEM CeO2 - La2O3
A. Correia dos Santos1 , M.O. Figueiredo2
1Chem. Dept., Citecmat, Faculdade
Ciências, Univ. Lisboa, Campo Grande, 1700 Lisboa, Portugal
2Cryst. Miner. Center, IICT, & CENIMAT, Mater.
Sc. Dept., FCT/UNL, 2825 Monte de Caparica, Portugal
Oxide systems MO2-M2O3 (M=REE) have been intensively studied lately due to the possibility of developing fluorite-type [1,2] and pyrochlore-type phases[3] with electronic and oxygen ionic conductivity, therefore suitable as solid electrolytes. A close and well known structural relationship exists between these two structure types. Pyrochlore - space group Fd3m, with 56 oxygens per unit cell - may be derived from the fluorite-type array (Fm3m, 4 oxygens per unit cell) through an ordering of vacant packing sites within the anionic array, giving rise to changes in the geometry of the available interstitial sites - regular cubic coordination fractionates into octahedral plus distorted cubic. In general, defective, disordered fluorite-type A study of the CeO2-La2O3 system was undertaken, the samples being synthesized by conventional solid state reaction using the component oxides. Special attention was given to the formation of fluorite-type solid solutions up to the 1:1 composition - the single phase Ce0.5 La0.5 O3.5. The formation process of this solid solution was studied as a function of temperature until 1250ºC by X-ray diffraction. A lattice constant larger than that of pure CeO2 (5.411Å) was found for the solid solution (5.5 The structural evolution when incorporating larger proportions of lanthanum oxide is commented and a discussion is presented on possible crystal chemical factors driving the development of a pyrochlore-type arrangement from the parent fluorite-type solid solution.