Ion diffusion-controlled processes in fluoride crystals

Abstract

Ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF 2 , BaF 2 and LiBaF 3 crystals (X-ray irradiated at 290 K) have been investigated by means of ionic conductivity and the correlated ionic thermally stimulated depolarisation current (TSDC), and radiation-induced optical absorption band thermal bleaching techniques at 290-650 K. It was found that under a DC field fluorides store large ionic space-charge. In CaF 2 , BaF 2 and LiBaF 3 , by using the ionic TSDC technique we were able to detect a series of the interstitial anion and/or anion vacancy delocalisation stages in the extrinsic ionic conductivity region. At least 4-6 wide and overlapping ionic TSDC peaks were observed. The activation energy values of the ionic conductivity and ionic TSDC coincide, or are very close: 1.313 ± 0.002 and 1.27 ± 0.02eV for CaF 2 , 1.040 ± 0.002 and 1.00 ± 0.02 eV for BaF 2 and 0.91 ± 0.04 and 0.780 ± 0.013 eV for LiBaF 3 , respectively. The correlated data of the ionic TSDC and thermal bleaching (of the F band and other radiation-induced absorption bands) suggest that above RT ion diffusion-controlled TSR processes take place in CaF 2 , BaF 2 and LiBaF 3 crystals. These processes are initiated and controlled by the anion defect thermal detrapping, migration and interaction with the trapped charges.