Hg2P2S6: A layered mercury hexathiodiphosphate (IV) with large birefringence

Birefringent materials are very important in the laser industry. By combining the hexathiodiphosphate(IV) [P2S6]4- unit and the highly polarizable Hg2+, a mercury hexathiodiphosphate(IV) Hg2P2S6 was designed and synthesized by using solid-state synthesis. Hg2P2S6 crystallizes in P 1¯ space group. It has a layered structure as in many hexathiodiphosphates(IV) such as ZnPS3, CdPS3. Its crystal structure, electronic structure and optical properties are investigated by experimental methods and first-principle calculations. Hg2P2S6 has a very large birefringence of 0.50 ​at 1064 ​nm and 0.48 ​at 2050 ​nm. Theoretical studies show the large birefringence comes from the synergistic effect of [P2S6]4- and [HgS4]6- groups with strong polarizability anisotropy.