Dilute Soft-Cation Anchoring Stabilizes γ-NaAsSe2 against Moisture While Preserving Giant Mid-Infrared Nonlinearity

Designing nonlinear optical materials that combine large second-harmonic generation (SHG) efficiency with long-term stability remains a challenge for infrared photonic applications. Chalcogenide crystals such as γ-NaAsSe2 (γ-NAS) have recently emerged as outstanding candidates, exhibiting an exceptionally high SHG susceptibility of ∼590 pm/V at 2 μm. Despite this exceptional SHG response, poor ambient stability has hindered practical use, largely because the labile Na/Se interchain environment is vulnerable to moisture-assisted degradation. Here, we address this limitation by combining Sb substitution at the As site, which stabilizes the polar γ framework, with dilute Ag substitution at the Na site to introduce stronger chalcogenophilic Ag-Se contacts without disrupting the electronic structure responsible for the large nonlinear response. Because phase analysis shows Ag has limited solubility in the γ-NAS lattice (≤2%), we selected γ-Na0.99Ag0.01As0.95Sb0.05Se2 for Bridgman crystal growth. Large single crystals remain stable under ambient conditions for 365 days while preserving a giant SHG response of ∼600 ± 15 pm/V, with stable SHG intensity confirmed during continuous ambient measurements. Dilute Ag substitution introduces more covalent Ag-Se contacts that act as local anchors between chalcogenide chains, suppressing moisture-assisted lattice opening while preserving the polar framework. Sb substitution stabilizes the polar γ polymorph, whereas dilute Ag substitution introduces more covalent Ag-Se contacts at the alkali-metal sublattice to improve ambient durability. Phase-matching calculations further suggest Type I phase-matchability. This work introduces dilute soft-cation anchoring as a chemical strategy for stabilizing moisture-sensitive polar chalcogenides while preserving the electronic structure responsible for large nonlinear optical responses.