A. Veispals

Colour centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF 3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within a radiation defect effectively created in LiBaF 3 single crystals. Following the Mollwo–Ivey relation we discuss investigations of optical dichroism, magnetic optical dichroism, as well as the assumptions regarding the accumulation kinetics of these absorption bands, the F centre being the main radiation defect created by X-rays in undoped LiBaF 3 crystals at RT.

Colour centres in LiBaF3

Abstract The X-ray-induced optical absorption (OA) and the spectra of magnetic circular dichroism (MCD) followed by optically detected EPR (ODEPR) have been investigated in undoped LiBaF3 crystals. According to ODEPR, the absorption bands in the spectral region 470–770 nm correspond to the F-type centres, whereas the absorption band at 420 nm is assigned to the hole centres. The redistribution of electrons to the thermal more stable F-type centres indicates that all types of colour centres are annealed in the course of the thermal release of electrons from F-type centres, accompanied by a thermostimulated luminescence.

F-type centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within the same radiation defect effectively created in LiBaF3. The Mollwo-Ivey relation as well as investigations of optical dichroism, magnetic circular dichroism and the assumptions regarding the accumulation kinetics of these absorption bands lead us to suggest that the F-type centre being the main radiation defect created by X-rays in LiBaF3 crystals at RT.

Photo- and thermostimulated processes in α-Al2O3

Abstract We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al 2 O 3 crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al 2 O 3 crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al 2 O 3 crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al 2 O 3 in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.

Self-trapped holes and recombination luminescence in LiBaF3 crystals

Abstract We investigated electron paramagnetic resonance (EPR) angular dependencies, recombination afterglow and thermostimulated luminescence of undoped LiBaF3 crystals, X-irradiated at low temperatures. EPR parameters of the F2− molecule oriented along the [1 1 0] direction have been obtained. Based on the value of the g-shift Δg=g⊥−gII=0.02, characteristic for the VK-centres in similar perovskites, we propose that we indeed observed the VK-centres, not the H-centres. X-irradiation below 170 K results in creation of a long-time temperature-independent afterglow due to the tunnelling recombination between close electron and hole centres. The F-type electron centres and the VK as well as probably O2− centres are proposed to be the tunnelling recombination partners, responsible for the 4.1 and 3.15 eV luminescence bands, respectively.

Ionic charge transport, thermoactivated delocalization and interaction with radiation-induced defects in LiBaF3 crystals

Abstract The ionic and ion-diffusion controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2 and LiBaF3 crystals have been investigated at 290–650 K by means of the ionic conductivity, ionic thermally stimulated depolarization current (TSDC) and X-ray induced optical absorption spectra TS bleaching (TSB) techniques. In the region of the extrinsic ionic conductivity a number of the ionic TSDC peaks (anion vacancy and interstitial detrapping) and correlated TSB stages have been detected for the first time. The TSR and TSB kinetics above RT in fluoride crystals (X-ray irradiated at RT) are initiated and controlled by the anion interaction with the radiation-induced defects, i.e., the anion diffusion controlled processes take place.

Thermal relaxation of colour centres in LiBaF3 crystals

Abstract Processes in LiBaF3 crystals caused by the thermal decay of F-type centres created by X-irradiation at room temperature have been examined. It is shown that the thermal decay of F-type centres results in the formation of two kinds of electron centres peaking at 630 nm and 740 nm differing in thermal stability. Weak TSL intensity, accompanying the decay of F-centres, also observed as well as the low value of the process activation energy suggest that due to the presence of moving anion vacancies a random walk of the F-centres occur. We propose that in course of the random walk of the F-centres both the aggregate F-centres are created and the annihilation with some complementary radiation defects take place.

EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.

Self-trapped holes in LiBaF3 crystals

Abstract Self-trapped holes (VK-centres) in x-irradiated LiBaF3 crystals were investigated by electron paramagnetic resonance (EPR), recombination afterglow and thermostimulated luminescence (TSL). After x-irradiation at 77K, an EPR of the self-trapped hole centre Vk (F2 −) oriented along the [110] axis is identified. The 19F hyperfine interaction parameters of Vk- centres estimated from EPR angular dependencies are: A II = 2520 MHz; A ⊥ = 200 MHz; the g-tensor parameters are: gII = 2.002 and g II = 2.024. X-irradiation at temperatures below 200K results in a creation of a long-term temperature-independent afterglow-tunnelling luminescence (TL), with main emission bands at ∼4.1 eV and ∼3.15 eV. The short wavelength TL bands are associated with the tunnelling recombination of the electron centre with the Vk-centre, with thermal stability estimated to be about 130K.

Ion diffusion-controlled processes in fluoride crystals

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.