From Phosphate Fluoride to Fluorophosphate: Design of Novel Ultraviolet/Deep-Ultraviolet Nonlinear Optical Materials for BePO3F with Optical Property Enhancement

Fluorine-containing compounds have stimulated the exploration of ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials. Alkali/alkaline-earth metal phosphates are one of the important potential systems as NLO materials, while the common small birefringence limits the phase-matching (PM) ability in the ultraviolet/deep-ultraviolet region. Herein, by applying a "fluorination synergy-induced enhancement of optical property" strategy, novel structures of phosphate fluoride/fluorophosphate in BePO3F with good thermodynamic/dynamic stability and promising NLO-related properties are discovered via performing crystal structure prediction combined with first-principles calculations. BePO3F-I–VI exhibit relatively large birefringence of 0.025, 0.048, 0.049, 0.049, 0.059, and 0.063 at 1064 nm, respectively. Simultaneously, BePO3F-I (Pc) is a new thermodynamically stable phosphate fluoride which possesses a wide band gap (Eg = 8.03 eV), large second-harmonic generation (SHG) coefficient (d11 = 0.67 pm/V, 1.7 × KDP), and the shortest PM wavelength of 292 nm. Other five thermodynamically metastable noncentrosymmetric (NCS) BePO3F structures (II–VI) belong to fluorophosphates and exhibit deep-ultraviolet PM wavelengths of 187, 183, 186, 188, and 196 nm. It reveals that the aligned nonbonding O 2p orbitals of [BeO2F2] and [PO4] units lead to a large SHG coefficient in the phosphate fluoride BePO3F-I. For fluorophosphates (BePO3F-II–VI), the synergy of [BeO3] planar units and [PO3F] units induces relatively large birefringence. Our research results provide an idea for exploring novel high-performance NLO materials.