CRYSTAL-STRUCTURE OF CE2O2.5S, AN OXIDIZED FORM OF CE2O2S, FROM NEUTRON AND X-RAY-POWDER DIFFRACTION

The structure of Ce2O2.5S, a form of Ce2O2S oxidized by exposure to ai r, was determined from combined X-ray and neutron powder diffraction d ata. The phase retains the trigonal symmetry and the <P(3)over bar m1> space group of the parent compound with cell parameters a = 3.96671(1 0) Angstrom and c = 6.8214(2) Angstrom and Z = 1. Refinements of 46 pa rameters by the Rietveld method, using 80 (X-ray) and 138 (neutron) re flections, led to R(p) = 0.102 and R(wp) = 0.129 (X-ray), R(p) = 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 oxyge n anions in the hexagonal prismatic bipyramidal cavity of the host str ucture. These additional oxygen atoms occupy a well defined off center ed 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.