Laporte Selection Rule Induced the Suppression of d-d Transitions and Vacancies Enhanced the Second Harmonic Generation in I 4 ¯ -MnGa2S4.

The d-d transitions lead to strong orbital hybridization near the band edge, decreasing the band gap (Eg) for magnetic transition metal chalcogenides. The small Eg seriously limits the high laser-induced damage threshold (LIDT) of the mid-infrared (IR) nonlinear optical (NLO) crystal. Herein, a quasi-centrosymmetric (quasi-CS) [MnS4] tetrahedron is proposed to suppress the d-d transitions by the Laporte selection rule in I 4 ¯ -MnGa2S4 for the first time, leading to the largest Eg of 3.43 eV in magnetic transition metal NLO sulfides. The partial density of states calculations indicate that the Mn-3d orbital hardly contributes to the optical transitions in [MnS4] tetrahedron. Meanwhile, the consistent arrangement of tetrahedral motifs and cation vacancies in defect chalcopyrite I 4 ¯ -MnGa2S4 improve the second harmonic generation (SHG) response. As a result, the I 4 ¯ -MnGa2S4 possesses a large SHG response of 2.1 × AgGaS2 (AGS) and a high polycrystalline LIDT of 8.15 × AGS. In addition, the isomorphic solid-solution I 4 ¯ -MnGa2SexS4- x (x = 1, 2, 2.8, and 4) exhibit an enhanced SHG response of 4.6 × AGS for I 4 ¯ -MnGa2Se4.