Ba2Ge2Te5: a ternary NLO-active telluride with unusual one-dimensional helical chains and giant second harmonic-generation tensors

Abstract

Non-centrosymmetric (NCS) metal chalcogenides are candidates for promising nonlinear optical (NLO) crystals in infrared (IR) bands. However, most research has been focused on sulfides and selenides, with few studies focused on tellurides. For the first time, we systematically investigated the linear-optical and NLO properties of a ternary NLO-active telluride, Ba₂Ge₂Te₅, at the experimental and theoretical levels. It adopts the NCS space group of Pna2₁ (no. 33) and features an unusual one-dimensional helical chain of [Ge₂Te₅]⁴⁻ separated by Ba²⁺. Ba₂Ge₂Te₅ exhibits a suitable birefringence (Δn) of 0.07, and a giant NLO coefficient d₃₃ of 286.8 pm V⁻¹ at a wavelength of 2050 nm (∼0.61 eV), much larger than that of the reference AgGaS₂ (18.2 pm V⁻¹). This work indicates that ethane-like [Ge₂Te₆] is an instructive NLO-active functional group that can be used to design novel and promising IR-NLO materials.