LiBaF3 Crystals Research Articles

Magnetooptical studies of defects and recombination luminescence in LiBaF3

Optically detected EPR investigations have been performed on the recombination luminescence (RL-EPR) of LiBaF3 crystal, X-irradiated at T=4.2K. RL-EPR lines of VK- centres were found, as well as further lines of a defect with S=12 and an axial g-tensor with its main axis along a [100] direction of the crystal.Measurements of the magnetic circular dichroism of the absorption (MCDA) have been performed on LiBaF3 crystals X-irradiated at two temperatures (4.2K and RT). After irradiation at T=4.2K, the main MCDA bands peak at 453 and 500nm, but after irradiation at T=300K, the main bands peak at 444 and 390nm, there is a change of the sign between the peaks in both cases. The MCDA-detected EPR (MCDA-EPR) consists of one broad EPR line in both cases and belong to electron trap centres. Analysis of half-widths of MCDA-EPR lines showed that both defects should have g-tensors with their axes along the [100] direction. This symmetry has to be expected for F-type centres in LiBaF3 crystals. The low-temperature electron centre has a more perturbed ground state as the RT centre.

Thermostimulated recombination processes in LiBaF3 crystals

The creation of radiation defects in LiBaF3 crystals at 10K and the processes of their thermostimulated recombination are investigated. The methods of optical absorption, thermal bleaching of color centers, thermostimulated luminescence and fractional glow technique are used. The radiation defects anneal in a multi-stage process accompanied with thermo-luminescence at 20, 46, 105, 130, 170, 210 and 270K. Differences in the optical absorption spectra measured before and after the TSL peaks are obtained and recombination parameters are determined. The TSL peak at 20K arises from the delocalization of H-centers. The presence of two TSL peaks related to VK-centers at 105 and 130K indicates that 60° and 90° migration hops occur.

Thermally and optically stimulated radiative processes in LiBaF3 crystals

In LiBaF3 crystals both valence–core transitions (5.4–6.5eV) and so-called self-trapped exciton luminescence (about 4.3eV) are important for practical application. Here, we present a study of 4.3eV luminescence under photo- and thermostimulation after X-irradiation of undoped LiBaF3 crystals at various temperatures. Optically stimulated luminescence as a result of electron recombination with both self-trapped holes and holes localized at some defects, were observed after X-irradiation below 130K and that of electron recombination with defect-localized holes was observed after X-irradiation above 130K. The spectra of thermo-stimulated luminescence (TSL) contain a broad band about 4eV related to the electron (high-energy side) or hole (low-energy side) recombination depending on TSL peak temperature.

Ion diffusion-controlled thermally stimulated processes in x-ray irradiated halide crystals

The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2, LiBaF3 and KBr crystals were investigated above 290 K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF2, BaF2, LiBaF3 and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal show that above 290 K the TSR processes are initiated and controlled by the ionic defect thermal detrapping, migration and interaction with the localised electronic and ionic charges and colour centres. The ion diffusion-controlled TSR processes take place in the above halide crystals.

Luminescent detectors of ionising radiation

At present in slow neutron imaging an active layer of an imaging plate IP contains a mixture of storage phosphors, usually BaFBr:Eu2+ used for imaging of X-rays, and a neutron converter material, usually Gd2O3, LiF.A novel Li-containing luminescent material perspective for a direct neutron conversion and storage is discussed. Irradiation of LiBaF3 crystals results in generation of Frenkel defect pairs and creation of F-type centres responsible for three absorption bands in UV-and visible spectral region. Because photo-stimulation in each of these absorption bands leads to bleaching of induced absorption, the F-type colour centres are convenient for storage of radiation dose. Photo-stimulated decay of F-centres causes recombination luminescence of impurity centres. Thermoactivated decay of F-type centres is governed by ionic process. The thermal stability of F-centres at RT and consequent material storage characteristics can be improved by doping of the LiBaF3 with heterovalent oxygen impurities. The obtained radiation energy storage, photo- and thermostimulated read-out characteristics justify that LIBaF3 is a suitable active media for imaging of slow neutrons.

Annealing of Radiation Defects in X-Irradiated LiBaF3

AbstractResults of application of the glow rate technique GRT for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in LiBaF3 crystals, pure and containing hetero-valence oxygen centers are presented. It is shown that depending on impurity composition in crystal two alternative mechanisms are involved in annealing of color centers. It is proposed that either the anion vacancy governed migration of F- centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F- type centers are responsible for the recombination of radiation defects above RT.

Ionic, electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF2 BaF2 and LiBaF3 crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF3 indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by Vk center detrapping and hole-diffusion controlled tunnel recombination within pairs like {Dn e−…Vk }.

Thermal relaxation of colour centres in LiBaF3 crystals

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.

Self-trapped holes in LiBaF3 crystals

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.

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

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.

F-type centres in LiBaF3 crystals

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.

Hole self-trapping and recombination in LiBaF3

We investigated the electron paramagnetic resonance (EPR), recombination afterglow and thermostimulated luminescence (TSL) of the X-irradiated LiBaF3 crystals. After X-irradiation at 80 K, an EPR of the self-trapped hole centre VK (F− 2) oriented along the [110] axis is identified. X-irradiation at temperatures below 200 K results in a creation of a long-term temperature-independent afterglow—tunnelling luminescence (TL), with main emission bands at 300, 370 and 430 nm. 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 130 K.

Colour centres in LiBaF3 crystals

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 a radiation defect effectively created in LiBaF3 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 LiBaF3 crystals at RT.