Prediction of deep-ultraviolet nonlinear optical material in borosilicate system: a case of RbSiB3O7

Abstract

Borates and their derivatives with nonlinear optical (NLO) effects are considered preferred materials for the generation of deep-ultraviolet (deep-UV) lasers. Despite the importance of this class of materials, only a small fraction has been explored, which greatly limits the discovery of new borate-based materials with deep-UV NLO performance. Herein, by performing crystal structure searches and first-principles properties calculations, we propose a deep-UV NLO borosilicate, RbSiB₃O₇, which was designed by introducing SiO₂ into RbB₃O₅. We successfully predicted a new thermo-dynamically stable phase, RbSiB₃O₇-I, along with several metastable phases (RbSiB₃O₇-II to RbSiB₃O₇-XI). All of these RbSiB₃O₇ structures feature distinctive [SiB₃O₁₀] fundamental building blocks (FBBs), each of which consists of one [SiO₄], one [BO₄], and two [BO₃] units. Compared with its parent compound RbB₃O₅, in which one [BO₄] and two [BO₃] units form [B₃O₇] rings, the introduction of the [SiO₄] unit transforms the [B₃O₇] FBB into [SiB₃O₁₀] FBB. From structural and physical perspectives, such a modification may yield a better balance among NLO-related properties. Remarkably, RbSiB₃O₇-V possesses a large band gap (∼7 eV), a large second-harmonic generation coefficient (∼1.5 × KH₂PO₄), and suitable birefringence (∼0.07 at 1064 nm), thereby enabling deep-UV phase matching. Considering that borosilicates still contain a vast unexplored chemical space, the present study may stimulate the exploration of new deep-UV NLO borosilicates with superior properties.