AAg₂MS₄ (A = Na, K, Rb; M = P, As) Nonlinear Optical Series with Unique Cation‐Regulated Highly Anisotropic [AgS₄] Hypertetrahedral Layered Framework

Abstract Commercial infrared (IR) nonlinear optical (NLO) materials are mainly from chalcopyrite‐type AMQ₂ (A = Ag, Zn, M = Ga, Ge, Q = S, Se, P) crystals such as AgGaS₂ (AGS), AgGaSe₂ (AGSe), and ZnGeP₂ (ZGP), in which the basic NLO‐active groups are MQ₄ tetrahedral units. However, the small anisotropic character of them critical for small birefringence hinders the practical applications in the available IR wavelength range. In this study, a novel strategy for modulating birefringence is proposed using a cation regulation‐driven compressed hypertetrahedral layer approach and a family of AAg₂MS₄ (A = Na, K, Rb; M = P, As) IR NLO crystals are successfully obtained. Remarkably, the title compounds increase the birefringence to 0.12–0.15, about three times larger than that of AGS (0.04), while they can maintain large bandgaps and strong NLO effects. In addition, they exhibit congruent‐melting properties, beneficial for bulk single crystal growth. The work offers a firsthand access for designing and fabricating high‐performance IR NLO materials.