Effect of electric field on optical properties of post gamma-irradiated lithium potassium sulphate crystals
Introduction
Significant attention is currently being paid to M1M2BX4 (M1=Li+, Na+, M2=K+, Cs+, Rb+ ions, NH4+, N2H5+ group and BX4=SO42−, SeO42−) crystals because of their physical properties, such as ferroelectricity, piezoelectricity and ionic conductivity. Furthermore, various phase transitions have been emphasized, particularly ferroelectric to ferroelastic phase transitions in some selenate and sulfates [1], [2], [3]. Moreover, the phase transition corresponding to different crystals can be understood within a common thermal, dielectric, optical and mechanical framework.
As an example of such compounds, LiKSO4 (namely LKS) crystal has been extensively studied during the last 2 decades. The details of such type of crystals and its structural changes have been established after a lot of X-ray studies (e.g. Kerppinen et al. [4]). It exhibits a hexagonal symmetry with polar point group 6 and space group p63 at room temperature [5]. In addition, it undergoes several structural phase transitions below [6], [7] and above room temperature [8]. Fortunately, most of these phases are sensitive to external fields. For example, external fields modify the incommensurate-wave-induced transition between paraelectric and both incommensurate and commensurate phases. External fields also allow for the existence of multicritical points, where several phase transition lines merge [9].
The optical absorption method provides information regarding the optically induced transitions and the variation in energy band gap after irradiation.
This stimulated our attention to exploit the effect of the external dc electric field on the optical properties of γ-irradiated LiKSO4 crystals.
Section snippets
Sample preparation
Lithium potassium sulphate (LiKSO4) was prepared by mixing Li2SO4 and K2SO4 in equimolar ratios. The mixture was ground thoroughly together, and then heated isothermally at 900°C in a platinum crucible for 5 h. The melt was cooled to room temperature, and then ground in a gate mortar. The details of the dynamical and slow evaporation methods were presented elsewhere [10]. The obtained crystals were untwined and of good optical quality. Samples were cut perpendicular to the b-axis into thin slab
Absorption coefficient
The absorption spectra of LKS single crystals have been investigated at photon energies near the fundamental absorption region, and the absorption coefficient (α) was calculated at different photon energies using the relationwhere A=ln (I/Io) is the measured optical density (absorbance) of the sample and d is its thickness. On the other hand, the optical energy gap Egopt is related to the absorption coefficient (α) through the following formula [11]:where ℏω is the
Conclusions
(1) The pronounced change in the optical parameters against the field intensity is observed at the range from 0 to 0.5 kV/cm, above which the optical parameters become nearly stable. This fact indicates that the nucleation and growth of new domains may not be ideally isotropic
(2) The changes in the optical intensity shown in Fig. 1b reflect qualitatively the field-induced dipole displacements, i.e. better alignment of the dipoles in the direction of the applied field. It can be seen that the
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2020, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and AtomsCitation Excerpt :According to our assumption, this excitation energy corresponds to the band gap of the LiKSO4 crystal, Eg ≈ 5.64 eV. The estimated band gap value approximately coincides with those previously experimentally determined or theoretically calculated [30–35]. When the excitation energy decreases from 5.64 eV (curve 4) down to 4.96 eV (curve 6), the energy approaches to the direct intracenter excitation of a Cu+ impurity ion.
Recombination luminescence in alkali metal sulfates
2016, Optical MaterialsCitation Excerpt :On the basis of the calculations made by the authors [6,7] the band gaps were estimated for LiKSO4 and K2SO4 crystals, which were found to be 5.8 eV and 8–9 eV, respectively. The results of calculations are confirmed by the measurements of the absorption spectrum of LiKSO4 crystal in the spectral range from 1.5 to 6.2 eV [8,9]. The research works on intrinsic luminescence of alkali metal sulfates show that when they are excited by X-rays, synchrotron radiation or ultraviolet photons, complex emission bands appear in the 2–4 eV spectral range.
Radiation defects in alkali metal sulfates irradiated by ultraviolet photons
2016, Radiation MeasurementsCitation Excerpt :Thus, on the basis of these results it can be assumed that the bandgap in sulphates of alkali and alkaline earth metals is about 4.4–4.5 eV. According to the calculations of the authors (Kityk et al., 1994) and the experimental data on the absorption spectra (Madi et al., 1998; El-Muraikhi, 2001), the band gap in LiKSO4 crystals can be estimated as 5.5–6 eV. According to the calculations [6.7] and the experimental data (Sholokh et al., 1985) free electron-hole pairs in the alkali metal sulfates can appear at photon energies of 5.1 eV, 6.9 eV and 10.5 eV.