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Molecules 2018, 23(4), 757;

Long-Time Relaxation of Stress-Induced Birefringence of Microcrystalline Alkali Halide Crystals

Department of Integrated Science and Technology, Faculty of Science and Technology, Oita University, Dannoharu, 700, Oita city 870-1192, Japan
Department of Mechanical Engineering, National Institute of Technology, Oita College, Maki, 1666, Oita city 870-0152, Japan
Author to whom correspondence should be addressed.
Academic Editor: Reiko Kuroda
Received: 26 February 2018 / Revised: 18 March 2018 / Accepted: 21 March 2018 / Published: 25 March 2018
(This article belongs to the Special Issue Recent Advances in Chiroptical Spectroscopy)
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Alkali halide single crystals are most commonly used as the diluent matrix in the tablet method or disk technique for spectroscopic measurements. However, stress-induced birefringence (SIB) of alkali halides as well as intrinsic birefringence manifest during the disk formation process. Thus, the true chiroptical measurement is disturbed by optical anisotropies (OA) containing SIB and intrinsic birefringence, except in the case of optical homogeneity. SIB is generally larger than intrinsic birefringence and has a value of several thousand millidegrees in the ultraviolet-visible wavelength range, although this varies with disk type. Here, to investigate the SIB origin, alkali halide crystals were examined using polarized light, X-ray diffraction, Fourier-transform infrared, and electron backscattering diffraction spectroscopic measurements. It was found that, after stress release, the SIB exhibited nonlinear long-time relaxation, which roughly converged within several hours, with the only time-invariant intrinsic birefringence remaining being due to OA. This behavior was strongly related to an increase in the quasi-amorphous domain and the generation of an air gap between the crystallite boundaries and their pellets. Further, a straightforward correlation was found between amorphization and an increase in the disk water content caused by deliquescence. Thus, the OA of alkali halide single crystals was found to have two different origins yielding intrinsic birefringence and SIB. View Full-Text
Keywords: alkali halides; tablet method; birefringence; chiroptical measurement alkali halides; tablet method; birefringence; chiroptical measurement

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Ueno, H.; Arakane, R.; Matsumoto, Y.; Tsumura, T.; Kitazaki, A.; Takahashi, T.; Hirao, S.; Ohga, Y.; Harada, T. Long-Time Relaxation of Stress-Induced Birefringence of Microcrystalline Alkali Halide Crystals. Molecules 2018, 23, 757.

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