Borates—Crystal Structures of Prospective Nonlinear Optical Materials: High Anisotropy of the Thermal Expansion Caused by Anharmonic Atomic Vibrations

Rimma Bubnova,Barbara Albert,Sergey Volkov,Stanislav Filatov

doi:10.3390/cryst7030093

Abstract

In the present study the thermal structure evolution is reviewed for known nonlinear optical borates such as β-BaB2O4, LiB3O5, CsLiB6O10, Li2B4O7, K2Al2B2O7, and α-BiB3O6, based on single-crystal and powder X-ray diffraction data collected over wide temperature ranges. Temperature-dependent measurements of further borates are presented for the first time: α-BaB2O4 (295–673 K), β-BaB2O4 (98–693 K), LiB3O5 (98–650 K) and K2Al2B2O7 (98–348 K). In addition to the established criteria for nonlinear optical (NLO) properties of crystals, here the role of the anisotropy and anharmonicity of the thermal vibrations of atoms is analysed as well as changes in their coordination spheres and the anisotropy of the thermal expansion of the crystal structure. Non-centrosymmetric borates, especially those that have NLO properties, often show distinct anisotropies for each cation in comparison to centrosymmetric borates. All considered NLO borates contain BO3 triangles, which are the principal cause of the strong anisotropy of the thermal expansion.

Highlights

In recent years, there has been extensive research and continuous development on second-order nonlinear optical (NLO) materials due to their potential applications
As discussed by us for the first time [47], “nonlinear optical effects might be caused by the anharmonicity of atomic thermal displacements that can be seen in thermal expansion tensor and in deviations from Gaussian shape of the probability density function of atomic thermal displacement factors”
There is growing evidence that the NLO properties of a material are related to the particular features of its atomic molecular structure

Summary

Introduction

There has been extensive research and continuous development on second-order nonlinear optical (NLO) materials due to their potential applications. The revelation of the NLO effect on quartz crystals by Blombergen in 1962 [1] and the development of solid lasers in the early. 1960s initiated huge progress in laser science and technology. In this context, the search for new NLO materials with optimized properties continues to be of special interest. Studies have shown that the second-order NLO properties of crystalline materials are closely related to their structures. There is growing interest in the search for new, prospective NLO materials due to the widening technical applications as a result of the replacement of gaseous and ionic laser sources with solid lasers

Methods

Discussion

Conclusion