From Centrosymmetric Ca₃Ge₂O₃S₄ to Noncentrosymmetric Ca₃Ge₂O₄Se₃: A Nonlinear Optical Ca-Based Oxyselenide

Adjusting the structure of compounds through an anion partial substitution strategy is an effective means to explore nonlinear optical (NLO) materials. Here, a new oxysulfide Ca₃Ge₂O₃S₄ (CGOS) crystallized in the centrosymmetric (CS) space group P2₁/c was synthesized by a high-temperature solid-solid reaction. Taking CGOS as the parent, the first quaternary NLO Ca-based oxyselenide Ca₃Ge₂O₄Se₃ (CGOSe) crystallized in the noncentrosymmetric (NCS) space group R3m was obtained by anion partial substitution. The 0D structure of CGOS consists of isolated [Ge₂O₃S₄] dimers, with Ca²⁺ ions located between dimers to balance the charge. The 0D structure of CGOSe consists of isolated and parallelly arranged [Ge₂O₄Se₃] dimers, with Ca²⁺ ions located between dimers to balance the charge. The structural evolution between CGOS and CGOSe is elucidated. CGOSe has the widest band gap of 3.02 eV in the AE₃Ge₂O₄Q₃ (AE = Ca, Sr, Ba; Q = S, Se, Te) system. Apart from the obvious NLO activity, CGOSe exhibits a high laser-induced damage threshold of around 3.2 × AgGaS₂. The theoretical calculation results show that the [Ge₂O₄Se₃] dimer is the main source of the SHG response. This work enriches the relatively less-explored Ca-based oxychalcogenides as NLO materials.