CaBO2F: A novel deep-UV structural template with high nonlinear optical performance induced by electron delocalization

Nonlinear optical (NLO) materials with deep-ultraviolet (deep-UV) phase-matching capability are of current interest owing to their applications in photoelectric technology. The anionic framework, including the microscopic unit, arrangement, and connection mode, is vital to optimizing the contradictory NLO performances. Herein, the polarizability characteristics of low-dimensional anionic frameworks were investigated. Results revealed that the oriented electron delocalization in a specific direction enhances the polarization and optimizes the NLO performance. A new borate fluoride system, namely CaBO2F, was designed via top-down structural prediction methods. In this system, CaBO2F-I, II, and III with low energy have one-dimensional (BO2)∞ anionic frameworks. The first-principles calculations show that CaBO2F-I, II, and III have wide deep-UV band gaps, suitable birefringence (0.076–0.108@1064 nm), and large second harmonic generation coefficients (2.1–2.7 × KH2PO4 (KDP)). Particularly, CaBO2F-I has a short phase-matching wavelength of 195 nm (Heyd-Scuseria-Ernzerhof (HSE06)), which is the first alkaline earth borate fluoride template with one-dimensional (BO2)∞ that reaches deep-UV phase-matching capability. This result offers a new guideline for exploring novel NLO materials.