From Monofluorophosphates A2PO3F to Difluorophosphates APO2F2 (A = alkali metal): Design of a Potential Deep-Ultraviolet Nonlinear Optical Materials System with a Shortened Phase-Matching Wavelength

Exploring new nonlinear optical (NLO) crystals for the output of deep-ultraviolet (DUV) lasers via the frequency conversion technique is of great interest. Monofluorophosphate is a new chemical system for exploring DUV NLO crystals while difluorophosphate is still lacking in research but attracting attention. Herein, difluorophosphate is proved as a new potential member of the DUV NLO materials system based on high-throughtput crystal structure prediction combined with first-principles calculations. Our high-throughtput screening identified 34 structures for difluorophosphate APO2F2 and monofluorophosphate A2PO3F (A = Li, Na, K, Rb, Cs) with good thermodynamical (meta)stability and promising NLO properties. Among them, six dynamically stable APO2F2 structures show DUV performance as the shortest second harmonic generation (SHG) phase-matching (PM) wavelength around 185–199 nm and SHG coefficient larger than or comparable to that of KH2PO4 (KDP). Notably, the APO2F2 system possesses enhanced birefringence and shortened SHG PM wavelength compared to the A2PO3F system. It reveals that dual-fluorine drives stronger P–O bonding electron density along the direction of nmax in PO2F2 than that in PO3F, which is responsible for the enhanced birefringence and shortened SHG PM wavelength. These results provide a new direction and insight for exploring DUV NLO crystals.