Melilite oxychalcogenide Sr2FeGe2OS6: a phase-matching IR nonlinear optical material realized by isomorphous substitution

Abstract

Transition-metal-based chalcogenides have recently emerged as greatly promising infrared nonlinear optical (IR-NLO) candidates due to their unique structural chemistries and rich optical properties. However, Fe-based IR-NLO chalcogenides with phase-matching (PM) features have not yet been reported. In this work, a new non-centrosymmetric (NCS) melilite oxychalcogenide, Sr₂FeGe₂OS₆, has been prepared by an isomorphous substitution method, and the relationships between the microscopic crystal structure and macroscopic NLO activity were systematically investigated. Sr₂FeGe₂OS₆ adopts the tetragonal space group of P21m (no. 113) and features a unique two-dimensional structure with Cairo pentagonal tiling layers formed by the alternating connection of [Ge₂OS₆] dimers and [FeS₄] tetrahedra via corner-sharing and with the charge-balanced Sr²⁺ cations between these layers. Excitingly, Sr₂FeGe₂OS₆ is the first Fe-based example capable of achieving PM in the IR-NLO chalcogenide system and displays an outstanding IR-NLO comprehensive performance, including a wide energy gap (E_g = 2.24 eV), sufficient second-harmonic-generation (SHG) efficiency (d_eff = 5.89 pm V⁻¹ at 2050 nm) and strong laser-induced damage threshold (LIDT = 14.42 MW cm⁻²). Deeper structural and theoretical analysis suggests that the ordered arrangement of NLO-active motifs, [Ge₂OS₆] dimers and [FeS₄] tetrahedra, makes significant contributions to the strong d_eff and large birefringence (Δn). This work not only demonstrates a PM Fe-based NCS material for the first time but also puts forward a new design avenue for high-performance IR-NLO materials.