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

Abstract

We report a new DUV transparent (<200 nm) noncentrosymmetric Na_1.5Rb_0.5PO₃F·H₂O compound crystallizing in the Pmn2₁ space group. This compound exhibits the second largest birefringence in the monofluorophosphate family, Δn_obv. = 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.