Structural investigation of oxygen insertion within the Ce2Sn2O7-Ce2Sn2O8 pyrochlore solid solution by means of in situ neutron diffraction experiments

The amazing oxygen exchange properties within the Ce2Sn2O7-Ce2Sn2O8 pyrochl ore solid solution were investigated by means of in situ neutron diffractio n experiments. The structural modification of the Ce2Sn2O7 phase heated und er oxygen up to 1000 degrees C was followed on the basis of the neutron dif fraction patterns, continuously collected with a 5 min counting rate at 2.5 29 Angstrom. The structure of the Ce2Sn2O8 pyrochlore phase has been previo usly determined at room temperature on the basis of the powder neutron diff raction pattern collected at 1.594 Angstrom. Contrary to the oxygen-interca lated zirconate pyrochlores, oxygen atoms preferentially occupy a 32e cryst allographic site, which is more distant from the Sn4+ cation than the other interstitial 8a site. Two kinds of local oxygen defects were identified wi thin the Ce2Sn2O8 - epsilon (0 less than or equal to epsilon less than or e qual to 0.05) pyrochlore at 400 degrees C. The most stable is a tetrahedron of oxygen atoms (32e site) around an oxygen vacancy (8b site), comparable to the Willis clusters found in UO2 + x. The other, lying around the 8a sit e, involves short oxygen-oxygen distances and constitutes a new type of oxy gen cluster around tin atoms. In addition to the successive disappearance o f these local defects from 400 degrees C to 700 degrees C, cerium deinserti on was detected above 500 degrees C. This leads to new cerium deficient Ce2 - xSn2Oy phases, which exhibit structural changes. The oxygen storage capa city (OSC) of the Ce2Sn2O7 pyrochlore was thus detailed by considering the successive insertion/deinsertion reactions and the Ce-Sn-O phase diagram.