La2ZnGa2S6O: a melilite-type transition metal oxysulfide achieving a well-balanced nonlinear optical behavior†

Abstract

A combination of multiple anions (S²⁻ and O²⁻) was developed as an effective method for exploring new, excellent nonlinear optical (NLO) oxysulfides with adjustable properties based on the flexible anion (S/O) ratio in their structures. It should be noted that mixed melilite-type oxysulfides, containing both alkaline earth (Ae) and trivalent lanthanide (Ln) metals, exhibit natural noncentrosymmetric (NCS) and disordered structures, demonstrating good NLO properties. Herein, we introduced low-coordination ZnS₄ to replace high-coordination AeS₈ in La-based melilite, thereby breaking the initial disordered structure and leading to the formation of a La₂ZnGa₂S₆O NLO material with an ordered structure. Property investigations showed that La₂ZnGa₂S₆O achieved a well-balanced NLO behavior, with a wide optical bandgap (3.0 eV) and an ultra-strong phase-matching second harmonic generation (SHG) response (1.9 × AgGaS₂). Among the melilite-type transition-metal oxysulfides, La₂ZnGa₂S₆O exhibited the largest powder SHG response, which was attributed to the synergetic contributions of LaS₇O, ZnS₄ and GaS₃O anionic groups, based on SHG-density analysis. This indicates that La₂ZnGa₂S₆O is a potential NLO candidate for frequency conversion applications, and the combination of transition and lanthanide metals in its structure provides a feasible pathway to design new large SHG oxysulfides.