Isolated centres versus defect associates in Sm3+-doped CeO2: a spectroscopic investigation

The interactions between Sm3+ and oxygen vacancies in CeO2 are probed by the use of tuneable laser excited time-resolved photoluminescence and Raman spectroscopies. It is found that Sm3+ (with doping concentrations of 0.1, 0.3, 1 and 5 wt%) substitutes largely for Ce4+ in sites with cubic symmetry and the corresponding emission is sensitized via the Ce4+-O(2-)charge-transfer band of CeO2. It is established from the photoluminescence spectra measured at long delay after the laser pulse that the local environment around cubic Sm3+ centres is not changed with concentration and ceria size. In addition to cubic symmetry Sm3+ centres, low-symmetry Sm3+ centres tentatively assigned to the Sm3+-oxygen vacancy associates of nearest-neighbour type are also observed. Their emission is preferentially excited via the weak f-f absorption transitions of Sm3+. A relatively strong concentration-induced quenching of Sm3+ 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 Sm3+ and Eu3+ in CeO2 are also compared on the basis of their emission spectra and decays.