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

Abstract

The amazing oxygen exchange properties within the Ce₂Sn₂O₇–Ce₂Sn₂O₈ pyrochlore solid solution were investigated by means of in situ neutron diffraction experiments. The structural modification of the Ce₂Sn₂O₇ phase heated under oxygen up to 1000 °C was followed on the basis of the neutron diffraction patterns, continuously collected with a 5 min counting rate at 2.529 Å. The structure of the Ce₂Sn₂O₈ pyrochlore phase has been previously determined at room temperature on the basis of the powder neutron diffraction pattern collected at 1.594 Å. Contrary to the oxygen-intercalated zirconate pyrochlores, oxygen atoms preferentially occupy a 32e crystallographic site, which is more distant from the Sn⁴⁺ cation than the other interstitial 8a site. Two kinds of local oxygen defects were identified within the Ce₂Sn₂O_{8 – ε} (0 ≤ ε ≤ 0.05) pyrochlore at 400 °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 UO_{2 + x}. The other, lying around the 8a site, involves short oxygen–oxygen distances and constitutes a new type of oxygen cluster around tin atoms. In addition to the successive disappearance of these local defects from 400 °C to 700 °C, cerium deinsertion was detected above 500 °C. This leads to new cerium deficient Ce_{2 – x}Sn₂O_y phases, which exhibit structural changes. The oxygen storage capacity (OSC) of the Ce₂Sn₂O₇ pyrochlore was thus detailed by considering the successive insertion/deinsertion reactions and the Ce–Sn–O phase diagram.