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

Hiroki Ueno,Takunori Harada,Yoshihisa Matsumoto,Tomoki Tsumura,Ryoga Arakane,Yasushi Ohga,Akihito Kitazaki,Toru Takahashi,Shotaro Hirao

doi:10.3390/molecules23040757

Abstract

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.

Highlights

Solid-state chiroptical spectroscopy has begun to attract considerable attention as a new analytical tool for many research fields involving solid-state chirality
It is clear that cubic KBr single crystals exhibit intrinsic birefringence even though these crystals constitute an isometric system
KBr disks has stronger angular dependence than that of single crystals, which is in agreement with experimental results that cubic single crystals possess intrinsic birefringence even though they belong to an isometric system [10]

Summary

Introduction

Solid-state chiroptical spectroscopy has begun to attract considerable attention as a new analytical tool for many research fields involving solid-state chirality. Careful polarization analysis is indispensable for true chiroptical measurements of chiral samples dispersed in an optically anisotropic matrix Such measurements are performed using modern chiroptical spectrophotometers such as ECD and circularly polarized luminescence spectrophotometers. These devices are based on polarization-modulation techniques, because it is necessary to couple non-ideal optics and electronics with strong OA, i.e., with nonchiral signals related to linearly polarized phenomena [1,11]. We thoroughly investigate the (intrinsic + extrinsic) OA mechanism inducing nonlinear SIB relaxation in the alkali halide disks used in matrix-disk methods, and we present an essential matrix-disk method for analyzing chiral signals for cases in which a dedicated solid-state chiroptical instrument is unavailable. When the OA after the relaxation of SIB is negligibly small compared with the net chirality signal, an artifact-free chiral signal can be obtained in the polarization-modulation spectroscopic measurements

Experimental

Results

Conclusions