Twins in YAl3(BO3)4 and K2Al2B2O7 Crystals as Revealed by Changes in Optical Activity

Johannes Buchen,Andreas Gross,Daniel Rytz,Steffen Dehmelt,Volker Wesemann

doi:10.3390/cryst9010008

Abstract

Many borate crystals feature nonlinear optical properties that allow for efficient frequency conversion of common lasers down into the ultraviolet spectrum. Twinning may degrade crystal quality and affect nonlinear optical properties, in particular if crystals are composed of twin domains with opposing polarities. Here, we use measurements of optical activity to demonstrate the existence of inversion twins within single crystals of YAl 3 (BO 3 ) 4 (YAB) and K 2 Al 2 B 2 O 7 (KABO). We determine the optical rotatory dispersion of YAB and KABO throughout the visible spectrum using a spectrophotometer with rotatable polarizers. Space-resolved measurements of the optical rotation can be related to the twin structure and give estimates on the extent of twinning. The reported dispersion relations for the rotatory power of YAB and KABO may be used to assess crystal quality and to select twin-free specimens.

Highlights

A series of borates with the general formula R Al3 (BO3 )4 (R = Y or rare earth element (REE))adopts the crystal structure of the carbonate mineral huntite, CaMg3 (CO3 )4 [1,2,3]
Inter- and extrapolating our results using these parameters in the dispersion relation (6) gave excellent agreement with literature data on the rotatory power of low-quartz, even at wavelengths in the deep UV where the rotatory power is strongly affected by dispersion and outside the spectral range accessible with the spectrophotometer (Figure 2)
We developed and tested a simple method to measure the rotatory power of optically active crystals at different wavelengths using a spectrophotometer with rotatable polarizers

Summary

Introduction

Adopts the crystal structure of the carbonate mineral huntite, CaMg3 (CO3 )4 [1,2,3]. In this structure type, the coplanar arrangement of [BO3 ]3− groups amplifies the intrinsic nonlinear dielectric susceptibility of the [BO3 ]3− group and combines it with strong birefringence and a spectral transparency that extends into the ultraviolet (UV) region [4,5,6]. In contrast to many other NLO borates, YAB is non-hygroscopic and hard (Mohs hardness 7.5 [2]) as compared to β-BaB2 O4 (BBO) [10], LiB3 O5. Albeit not a huntite borate, the material K2 Al2 B2 O7 (KABO)

Methods

Results

Discussion

Conclusion