Isolated centres versus defect associates in Sm super(3+)-doped CeO sub(2): a spectroscopic investigation

The interactions between Sm super(3+) and oxygen vacancies in CeO sub(2) are probed by the use of tuneable laser excited time-resolved photoluminescence and Raman spectroscopies. It is found that Sm super(3+) (with doping concentrations of 0.1, 0.3, 1 and 5 wt%) substitutes largely for Ce super(4+) in sites with cubic symmetry and the corresponding emission is sensitized via the Ce super(4+)-O super(2-) charge-transfer band of CeO sub(2). It is established from the photoluminescence spectra measured at long delay after the laser pulse that the local environment around cubic Sm super(3+) centres is not changed with concentration and ceria size. In addition to cubic symmetry Sm super(3+) centres, low-symmetry Sm super(3+) centres tentatively assigned to the Sm super(3+)-oxygen vacancy associates of nearest-neighbour type are also observed. Their emission is preferentially excited via the weak f-f absorption transitions of Sm super(3+). A relatively strong concentration-induced quenching of Sm super(3+) emission was inferred from the decrease in the average emission lifetimes from 2.1 ms (0.1 wt%) to 0.87 ms (5 wt%). The local environments of Sm super(3+) and Eu super(3+) in CeO sub(2) are also compared on the basis of their emission spectra and decays.