Vacancy-Driven Tetrahedral Distortion in Defect Diamond-Like Selenides for Exceptional Second-Order Nonlinearity

Diamond-like (DL) chalcogenides have emerged as a highly promising structural framework for infrared nonlinear optical (NLO) materials due to their inherently non-centrosymmetric structures and highly oriented tetrahedral units. Herein, a family of defect DL selenides AIIBIII2Se4 (AII = Zn, Cd; BIII = Al, Ga, In) was systematically investigated by integrating theoretical calculations and experimental characterization. These compounds crystallize in the tetragonal I4̅ space group, adopting a defect DL structure derived from infrared benchmark AgGaS2 (AGS), incorporating cation vacancies and highly distorted [BIIISe4] tetrahedral units, which significantly enhance microscopic second-order polarizability. Experimental results demonstrate that these defect DL selenides exhibit strong second-harmonic generation (SHG) intensities and tunable band gaps. Remarkably, ZnIn2Se4 demonstrates a considerable SHG response, approximately 5.5 times that of the commercial benchmark AGS with phase-matching behavior. First-principles calculations further elucidate that their effective NLO coefficients d14 range from 16.44∼34.04 pm/V (calculated by sum over-states method) and 18.49∼35.11 pm/V (DFPT method), respectively, which are attributed to the pronounced bond-angle distortions within the [BIIISe4] tetrahedra, as well as the Se 4p lone-pair-like electrons. This work confirms that vacancy-driven tetrahedral distortion engineering is a potential design strategy for high-performance infrared NLO crystals.