A Congruent‐Melting Mid‐Infrared Nonlinear Optical Vanadate Exhibiting Strong Second‐Harmonic Generation

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Study of mid‐infrared (mid‐IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second‐harmonic generation (SHG) coefficient d ij and laser‐induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs 4 V 8 O 22 (CVO) featuring a quasi‐rigid honeycomb‐layered structure with [VO 4 ] and [VO 5 ] polyhedra aligned parallel. CVO possesses a wide IR‐transparent window, high LIDT, and congruent‐melting behavior. It has very strong phase‐matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First‐principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V 4 O 11 ] ∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.