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

Exploringnew nonlinear optical (NLO) crystals for the output ofdeep-ultraviolet (DUV) lasers via the frequency conversion techniqueis of great interest. Monofluorophosphate is a new chemical systemfor exploring DUV NLO crystals while difluorophosphate is still lackingin research but attracting attention. Herein, difluorophosphate isproved as a new potential member of the DUV NLO materials system basedon high-throughtput crystal structure prediction combined with first-principlescalculations. Our high-throughtput screening identified 34 structuresfor difluorophosphate APO(2)F(2) and monofluorophosphateA(2)PO(3)F (A = Li, Na, K, Rb, Cs) with good thermodynamical(meta)stability and promising NLO properties. Among them, six dynamicallystable APO(2)F(2) structures show DUV performanceas the shortest second harmonic generation (SHG) phase-matching (PM)wavelength around 185-199 nm and SHG coefficient larger thanor comparable to that of KH2PO4 (KDP). Notably,the APO(2)F(2) system possesses enhanced birefringenceand shortened SHG PM wavelength compared to the A(2)PO(3)F system. It reveals that dual-fluorine drives stronger P-Obonding electron density along the direction of n (max) in PO2F2 than that in PO3F, which is responsible for the enhanced birefringence andshortened SHG PM wavelength. These results provide a new directionand insight for exploring DUV NLO crystals.