Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap

The achievement of balanced performance with a strong second-harmonic generation (SHG) response, proper birefringence, and wide band gap concurrently is a crucial but challenging task in infrared nonlinear optical (IR-NLO) crystals. Here, we theoretically confirmed that the heteroanionic oxysulfide tetrahedron would produce improved polarizability anisotropy and quadratic hyperpolarizability compared with the monoanionic oxide or sulfide tetrahedra. When this anion-mixing strategy was applied, melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as a promising IR-NLO material system. Remarkably, compared with the monoanionic melilite oxides, these compounds exhibited unbiased performances of proper birefringences (0.106–0.143), strong SHG responses (>10× melilite oxide Ba2CdGe2O7, 1.3–2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable ability, wide band gaps (2.95–3.15 eV), and large laser-induced damage thresholds (LIDTs, 5.6–13.4× AGS). The high structure tolerance of melilite offers a great possibility to improve the SHG response via tuning the orbital composition and hybridization near band edges. This work provides an effective approach for the design of high-performance IR-NLO materials.在红外非线性光学（IR-NLO）晶体领域，同时实现兼具强二阶谐波产生（second-harmonic generation, SHG）响应、合适双折射与宽禁带的平衡性能，是一项关键且极具挑战性的任务。本研究从理论上证实，相较于单阴离子氧化物或硫化物四面体，杂阴离子氧硫化物四面体可展现出更优异的极化各向异性与二阶超极化率。将该阴离子混合策略付诸实践后，本研究成功开发出黄长石型氧硫化物体系，其包含四类典型成员：A₂GeGa₂OS₆（A=Ca、Sr）与Sr₂MGe₂OS₆（M=Zn、Cd），是一类极具潜力的IR-NLO材料体系。值得注意的是，相较于单阴离子黄长石型氧化物，这类化合物展现出协调均衡的综合性能：合适的双折射值（0.106~0.143）、强SHG响应（为黄长石型氧化物Ba₂CdGe₂O₇的10倍以上，在1570 nm波长下为AgGaS₂（AGS）的1.3~2.1倍）且具备相位匹配能力、宽禁带（2.95~3.15 eV）以及高激光诱导损伤阈值（laser-induced damage threshold, LIDTs，为AGS的5.6~13.4倍）。黄长石结构所具备的高结构容忍度，为通过调控能带边附近的轨道组成与杂化来提升SHG响应提供了极大可能。本研究为高性能IR-NLO材料的设计提供了一条高效可行的途径。