Elsevier

Materials Letters

Volume 51, Issue 1, October 2001, Pages 19-26
Materials Letters

Effect of electric field on optical properties of post gamma-irradiated lithium potassium sulphate crystals

https://doi.org/10.1016/S0167-577X(01)00258-0Get rights and content

Abstract

Irradiation of lithium potassium sulphate crystals (LiKSO4) was carried out by 60Co-gamma rays at a dose 150 kGy. Post irradiation, external direct current (dc) electric field was applied perpendicular to ferroelastic plane. The field intensity was varied between 0 and 10 kV/cm. From measurement of ultraviolet (UV) absorption versus wavelength (A(λ)), optical band gap under various field intensity were deduced with Mott's plot. The pronounced change in the optical parameters against the field intensity is observed at the range from 0 to 0.5 kV/cm. This was attributed to the better alignment of the dipoles in the direction of the applied field. The obtained results were explained according the biasing inner electric field produced by γ-radiation, and subsequently stabilized by the external electric field, indicating the presence of interband transitions in such 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 relationα=Ad,where 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]:(αℏω)=B(ℏω−Egopt)n,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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