Dielectric relaxations in undoped, Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu3Al5O12, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz-1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion-anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O−- and F+-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV-visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.