[C(NH2)3]3PO4·2H2O: A new metal-free ultraviolet nonlinear optical phosphate with large birefringence and second-harmonic generation response

[PO4]3− possesses small microscopic second-order susceptibility and polarizability anisotropy, which inherently result in weak second-harmonic generation (SHG) and small birefringence. Herein, a new noncentrosymmetric phosphate, [C(NH2)3]3PO4·2H2O, was successfully designed and synthesized by a solution evaporation method. [C(NH2)3]3PO4·2H2O without metal ions is composed of planar conjugated guanidine cations and PO4 tetrahedrons, which are connected by hydrogen bonds, forming a three-dimensional network. Physical property measurements indicate that the title compound displays an SHG response of 1.5 × KH2PO4 (KDP) and a larger birefringence (0.055@546.1 nm) compared with most reported ultraviolet nonlinear optical phosphates. Furthermore, first-principles calculations reveal that the π-conjugated planar [C(NH2)3]+ cations and [PO4]3− groups are responsible for its excellent linear and NLO properties.