Ba6In2Ge2Te15: a THz birefringent material with an intriguing quasi-[Te5]4− chain possessing large optical anisotropy and an ultrawide transmission range

Abstract

Exploring mid- and far-infrared (MFIR) and even terahertz (THz) birefringent crystals with large optical anisotropy remains a great challenge. Herein, a novel THz birefringent material Ba₆In₂Ge₂Te₁₅, which contains the intriguing quasi-[Te₅]⁴⁻ chain, has been successfully synthesized and investigated. This compound displays an ultrawide transparent region (1.3–40.2 μm), exhibiting high transmittance from NIR to MFIR and even THz regions. Moreover, the birefringence of Ba₆In₂Ge₂Te₁₅ equals 0.377@30–42 μm (7.1–10 THz) and 0.42@2.09 μm, which are larger than those of most of the reported chalcogenides, both properties surpassing commercial birefringent crystals YVO₄ and TiO₂. Furthermore, the first-principles calculations demonstrate that the large optical anisotropy originates from the non-bonded p orbitals of Te atoms in the quasi-[Te₅]⁴⁻ chains. This work might shed light on the follow-up design and exploration of THz birefringent materials.