Crystal structure of Ce2O2.5S, an oxidized form of Ce2O2S, from neutron and X-ray powder diffraction

The structure of Ce 2 O 2.5 S, a form of Ce 2 O 2 S oxidized by exposure to air, was determined from combined X-ray and neutron powder diffraction data. The phase retains the trigonal symmetry and the P3m1 space group of the parent compound with cell parameters a = 3.96671(10) A and c = 6.8214(2) A and Z = 1. Refinements of 46 parameters by the Rietveld method, using 80 (X-ray) and 138 (neutron) reflections, led to Rp = 0.102 and R wp = 0.129 (X-ray), Rp = 0.063 and R wp = 0.048 (neutron), R p = 0.086 and R wp = 0.113 (global). The oxidation process is shown to correspond to an insertion of oxygen anions in the hexagonal prismatic bipyramidal cavity of the host structure. These additional oxygen atoms occupy a well defined off centered position of the cage, thereby forming short Ce-O and O-O distances. However, due to the disordered nature of the structure, no precise O-O distances and therefore phase charge balance information have been established.