Optical and structural properties of Eu2+ doped BaBrI and BaClI crystals

Abstract

The work is necessitated by search for new materials to detect ionizing radiation.The rare-earth ions doped with ternary alkali earth-halide systems are promising scintillators showing high efficiency and energy resolution. Some aspects of crystal growth and data on the structural and luminescence properties of BaBrI and BaClI doped with low concentrations of Eu 2+ ions are reported. The crystals are grown by the vertical Bridgman method in sealed quartz ampoule. New crystallography data for BaClI single crystal obtained by single crystal X-ray diffraction method are presented in this paper. Emission, excitation and optical absorption spectra as well as luminescence decay kinetics are studied under excitation by X-ray, vacuum ultraviolet and ultraviolet radiation. The energies of the first 4f-5d transition in Eu 2+ and band gap of the crystals have been obtained. We have calculated the electronic band structure of the crystals using density functional theory as implemented in the Ab Initio. Calculated band gap energies are in accord with the experimental estimates. The energy of gaps between the occupied Eu 2+ 4f level and the valence band top are predicted. In addition, positions of lanthanide energy levels in relation to valence band have been constructed using the chemical shift model.Recently the research focus is shifted to the study of mixed halide compounds due to their superior light yield . 6 In a number of barium dihalides BaFI-BaClI-BaBrI-BaBrCl, the scintillation properties have been studied for Eu-doped BaFI, BaBrCl and BaBrI. 7-11 Despite their excellent properties, experimental data on optical absorption and excitation spectra in spectral region of 4f-5d and band to band transitions are scarce because single-crystals doped with high concentrations of Eu 2+ ions (more than 5 mol. %) were used. When measuring high concentration doped samples, the inner filter effects can be observed. These include reabsorption and non uniform excitation throughout the sample. These effects dramatically change the shape of excitation spectrum. Therefore, estimation of the lowest energy of 4f-5d transitions in Eu-doped BaBrI given in 8,10 is not correct. Furthermore, the experimental determination of band gap in these crystals is not possible due to high absorption related to allowed 4f 6 5d 1 → 4f 7 transitions in Eu 2+ ions. At this moment, the energy of band gap of the mixed halide compounds is based on theoretical estimates.We investigate luminescence, electrical and structural properties of undoped BaBrI, BaBrI-0.05 mol.% Eu 2+ and BaClI-0.1 mol.% Eu 2+ crystals. Absorption, excitation and emission spectra, photoluminescence decay time constants, dielectric properties and pulsed height spectra are presented. The vacuum referred binding (VRBE) energy diagram is constructed in conformity with density functional study. It displays the electron binding energy in the ground and excited state levels of all divalent and trivalent lanthanides ions in BaBrI and BaClI crystals. Methodology Growth and Structural Characteriza...

Role of electron and hole centers in energy transfer in BaBrI crystals

“…As shown in the inset of Fig. 1, efficiency of F-centers creation decreases during cooling at temperatures about 100 K where intensity of STE band emission grows up [4,5]. This anticorrelation behaviour is similar to observed in alkali halides crystals [8].…”

“…2, curve 1. The band peaked at about 3 eV in PSL spectrum is attributed to 5d-4f emission of Eu 2+ ions [4]. The excitation spectrum of this PSL is given in Fig.…”

“…Eu-doped crystals and undoped crystals were grown by Bridgman technique reported in [4]. To study transformation of one type of F-centers into another, depending on the preparation conditions of the BaBrI we applied a Czochralski crystal growth technique.…”

“…2. PSL peak at about 3.9 eV is attributed to STE exciton emission [4]. Photostimulation spectrum of exciton related PSL shows wide peak at about 2 eV (curve 4, Fig.…”

“…BaBrI optically excited decay curves were fitted with only one single exponential decay component [4,16]. However, the decay curve under x-ray excitation was well approximated with three exponential decay components [16].…”