Hg[CS(NH2)2]4(SiF6): a fluorosilicate crystal with large birefringence achieved via multi-functional group modification

Abstract

Hexafluorosilicates are known for their short ultraviolet cutoff edges and wide band gaps. However, the low anisotropy of the SiF₆ groups has limited their application in optical functional materials. To address this, a method is proposed herein to enhance the birefringence of hexafluorosilicate through the incorporation of multiple functional groups. Specifically, thiourea CS(NH₂)₂ ligands and highly polarizable Hg²⁺ ions were introduced into the hexafluorosilicate system, leading to the successful synthesis of the first Hg-based hexafluorosilicate birefringent crystal, Hg[CS(NH₂)₂]₄(SiF₆). The structure comprises SiF₆ octahedra interconnected with zero-dimensional Hg[CS(NH₂)₂]₄ clusters via hydrogen bonding. The calculated birefringence at 546 nm is 0.137, which is consistent with the experimental value of 0.145. This performance is comparable to that of the commercial birefringent material α-BaB₂O₄ (0.122@546 nm) and significantly surpasses most previously reported inorganic hexafluorosilicates. This work provides a viable method for improving the birefringence of hexafluorosilicate compounds.