Ba2HgTe5: a Hg-based telluride with giant birefringence induced by linear [HgTe2] units

In this work, Ba2HgTe5, as the first compound identified in the Ba/Hg/Te system, has been successfully synthesized via a solid-state reaction. It crystallizes in the orthorhombic space group pnma and exhibits a 0D structure composed of linear [HgTe2] units and V-shaped [Te3] units arranged in an alternating manner. The experimental and theoretical calculation results indicate that Ba2HgTe5 is a potential IR birefringent material, possessing a moderate band gap (1.28 eV), giant optical anisotropy (Δncal = 0.643@2090 nm), a low melting point (576 °C), and congruent-melting behavior. The results of the structure–property relationship reveal that the band gap of Ba2HgTe5 is primarily defined by the Te p orbitals in the [Te3] unit. Further, the analysis of the response electron distribution anisotropy (REDA) illustrates that the large birefringence of Ba2HgTe5 is induced mainly from the linear [HgTe2] unit (about 90% contribution). This work demonstrates for the first time the rationality of the [HgTe2] group as the gene for designing IR birefringent materials.