An effective pathway to design and synthesize UV birefringent crystals via rational assembly of π-conjugated [CO3]2− and [NO3]− triangles

Abstract

Planar MO₃ (M = B, C, N) units have frequently been considered important structural components of novel birefringent crystal materials. An efficient approach for constructing new functional crystals is to simultaneously assemble multiple structural motifs together. Two compounds, Na₃Rb₆(CO₃)₃(NO₃)₂X·6H₂O (X = Br and Cl), were synthesized by the integration of three kinds of anionic groups. More interestingly, the [CO₃]²⁻ and [NO₃]⁻ groups in Na₃Rb₆(CO₃)₃(NO₃)₂X·6H₂O are all coplanar with the aid of [NaO₇]¹³⁻ polyhedra, which can enhance the anisotropic polarizability. Na₃Rb₆(CO₃)₃(NO₃)₂X·6H₂O have a large theoretical birefringence of ∼0.165 at 1064 nm and possess a short UV cut-off edge of ∼230 nm. Additionally, the two compounds exhibit good crystal growth habits. These properties illustrate that Na₃Rb₆(CO₃)₃(NO₃)₂X·6H₂O are promising UV birefringent crystals.