Na1.5Rb0.5PO3F·H2O: synthesis, properties, and stepwise reconstruction of the hydrogen bond network

We report a new DUV transparent (<200 nm) noncentrosymmetric Na1.5Rb0.5PO3F·H2O compound crystallizing in the Pmn21 space group. This compound exhibits the second largest birefringence in the monofluorophosphate family, Δnobv. = 0.04 at 546 nm, which is attributed to the nearly uniform alignment of the P–F bonds. The SHG response is observed to be 0.55 × KDP at 1064 nm. More interestingly, the precise substitution of NH4+ with Na+/Rb+ cations in the well-known structure (NH4)2PO3F·H2O leads to a stepwise reconstruction of the hydrogen bond networks, which eventually alters the crystal symmetry and the optical properties.