Melilite oxychalcogenide Sr2FeGe2OS6: a phase-matching IR nonlinear optical material realized by isomorphous substitution
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 (Eg = 2.24 eV), sufficient second-harmonic-generation (SHG) efficiency (deff = 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 deff 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.
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