Designing a Family of Aluminum-Containing Fluoroborate Crystals with Enhanced Birefringence and Second-Harmonic Generation Coefficients Based on the First-Principles Methods

Rb3Al3B3O10F (or RbAlBO3.33F0.33) is an F-deficient crystal grown at high temperature and reported to be a potential ultraviolet or deep ultraviolet (DUV) nonlinear optical crystal. However, the systematic study of all possible crystal structures and optical properties of MAlBO3F (M = K, Rb, and Cs) with the ideal stoichiometric ratio has never been reported. In the present work, a family containing 16 different crystal structures of MAlBO3F (M = K, Rb, and Cs) with large birefringence and second-harmonic generation (SHG) coefficients was predicted by the first-principles method. All these structures satisfy dynamic stability and Born elastic stability. The maximum birefringence of MAlBO3F (M = K, Rb, and Cs) is 0.07, which is comparable to the birefringence of KBe2BO3F2. The maximum SHG coefficient of MAlBO3F (M = K, Rb, and Cs) is -0.68 pm/V, which is about 1.74 times that of KDP (d(36)(KDP) = 0.39 pm/V). In addition, the potential correlation between the birefringence of MAlBO3F (M = K, Rb, and Cs) and the stacking modes of the optical functional units (BO3 and AlO3F or AlO3F2 coordination polyhedra) in the crystal has been revealed. This provides us with useful clues for the search of DUV M alpha Al beta B gamma O delta F epsilon crystals with large birefringence.

Automated summary

In this study, the crystal structures and optical properties of MAlBO3F (M = K, Rb, Cs) were systematically investigated using first-principles method, predicting 16 different crystal structures with potential applications in deep ultraviolet nonlinear optical crystals. The maximum birefringence and second-harmonic generation coefficients were found to be comparable or higher than those of known nonlinear optical crystals, providing useful clues for the search of new materials with desired optical properties.