AXHg3P2S8 (A = Rb, Cs; X = Cl, Br): New Excellent Infrared Nonlinear Optical Materials with Mixed‐Anion Chalcohalide Groups of Trigonal Planar [HgS2X]3− and Tetrahedral [HgS3X]5−

Abstract A series of nonlinear optical (NLO) chalcohalides AXHg 3 P 2 S 8 (A = Rb, Cs; X = Cl, Br), which contain various Hg‐based building units including mixed‐anion trigonal planar [HgS 2 X] 3− and tetrahedral [HgS 3 X] 5− , are successfully synthesized. These compounds display good water resistance, wide infrared transparent region (0.42–17 µ m), high laser‐induced damage thresholds (4.5–6 × AgGaS 2 ), and strong phase‐matchable second harmonic generation (SHG) response close to AgGaS 2 . The mixed‐anion chalcohalide groups serving as functional building units in NLO materials are studied for the first time and proven to be very effective. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG efficiency and large bandgap.