From Pb(H2C3N3O3)(OH) to Pb(H2C3N3O3)F: Homovalent Anion Substitution-Induced Band Gap Enlargement and Birefringence Enhancement

Birefringent materials capable of modulating the polarization of light have attracted intensive studies because of their wide utilization in optical communication and the laser industry. Herein, two new lead(II)-based cyanurates, namely, Pb(H2C3N3O3)X (X = OH, F), were synthesized by hydrothermal methods, and the first halogen-containing metal cyanurate Pb(H2C3N3O3)F was successfully obtained by the rational substitution of a homovalent anion. Pb(H2C3N3O3)X (X = OH, F) belong to space group P1(-), and their structures display a neutral [Pb(H2C3N3O3)X] (X = OH, F) layer. The Pb2+ ions in Pb(H2C3N3O3)(OH) are interconnected by hydroxyl groups and oxygen atoms of cyanurate anions into a 1D [PbO(OH)](-) chain, whereas the Pb2+ ions in Pb(H2C3N3O3)F are interconnected by F- anions and oxygen atoms of cyanurate anions into a 2D [PbOF](-) layer. The pi-pi interactions between adjacent hydroisocyanurate rings and the hydrogen bonds between neighboring neutral layers provide additional stability to the structures. Luminescent studies show that Pb(H2C3N3O3)(OH) and Pb(H2C3N3O3)F emit yellow-green and blue light, respectively. Theoretical calculations unveiled their birefringences of 0.079 and 0.203@1064 nm and their band gaps of 3.96 and 4.96 eV, respectively, for OH- and F- containing materials. Obviously, the substitution of OH- by F- with the largest electronegativity can simultaneously improve both the birefringence and band gap.