EPR characterization of O- and trapped electrons in 20% SrF2/SmOF

An X-ray diffraction study of the 20% SrF2/SmOF catalyst has revealed the presence of cubic SrF2 and rhombohedral SmOF crystal phases. The lattice of the SrF2 crystal phase was distorted. Cationic and anionic exchanges may have occurred to cause such deformity. When the 20% SrF2/SmOF sample was treated in helium at 1073 K, EPR signals corresponding to O- centres were observed. Hydrogen or ethane treatment of the sample at 973 K or above resulted in the detection of superhyperfine structures corresponding to the presence of trapped electrons. No similar EPR signals were observed on the SmOF sample. We suggest that these EPR-detectable species were located in the SrF2 lattice and existed largely in the bulk. The trapped electrons were stable in oxygen up to 673 K while O- centres were stable in ethane or hydrogen up to a temperature as high as 873 K. We speculate that the migration of O- centres and trapped electrons is more arduous in fluoride than in oxide. Hence the temperature for the combination of O- centres and trapped electrons as well as for the migration of O- centres and trapped electrons from the bulk towards the surface is higher in the former than in the latter.