Colour centres in electron irradiated mgo

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV-visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F(+) centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3-0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Point defects induced in yttria-stabilized zirconia by electron and swift heavy ion irradiations

We present an extensive study of point-defect creation in yttria-stabilized zirconia (ZrO(2):Y) exposed to 2.5 MeV electrons and various heavy ions (from C to U) covering an energy range from 100 MeV to several GeV. A synthesis of results from UV-visible optical absorption spectroscopy and electron paramagnetic resonance spectroscopy is provided with special emphasis on the respective roles of elastic collisions and electronic excitations. The colour centre production and recovery are the main focus in this survey. It is concluded that F(+)-type centres (involving singly ionized oxygen vacancies) are produced by elastic-collision processes. The large threshold displacement energy and defect volume hint that these colour centres might actually be small paramagnetic oxygen vacancy clusters, most probably divacancies (i.e. F(2)(+) centres). Such a picture is consistent with the (100) axial symmetry, inhomogeneous broadening of the optical absorption band, lack of hyperfine splitting, and weak spin-lattice coupling found for this defect.