Synthesis, crystal structure and optical properties of a new fluorocarbonate with an interesting sandwich-like structure

Abstract

A new fluorocarbonate, Na₃Zn₂(CO₃)₃F, was synthesized using a subcritical hydrothermal method. Na₃Zn₂(CO₃)₃F crystallizes in the space group C2/c with a sandwich-like framework in which the stacked [Zn(CO₃)]_∞ layers are connected with one another by bridging F atoms and [CO₃] groups alternately. Interestingly, each Zn atom is surrounded by one F atom and four O atoms, forming a distorted [ZnO₄F] trigonal bipyramid, which is observed for the first time in the carbonate system. Na₃Zn₂(CO₃)₃F has high transparency in a wide spectral region ranging from UV to mid IR with a short ultraviolet absorption edge (∼213 nm). First-principles calculations revealed that Na₃Zn₂(CO₃)₃F possesses a large birefringence (Δn = 0.11, λ = 589 nm), which is mainly contributed by the coplanar arrangement of [CO₃] groups in the ab plane. Na₃Zn₂(CO₃)₃F might find applications as a UV birefringence crystal.