Enlargement of Bandgap and Birefringence in Nonlinear Optical Alkali‐Metal Sulfate Crystals by the Substitution of Asymmetrical Non‐π‐Conjugated Cation

Abstract Wide bandgap and large birefringence, as well as strong second harmonic effect, are important but often mutually restraint prerequisites for deep‐ultraviolet (DUV) nonlinear optical (NLO) crystals, a type of photoelectronic materials that play an indispensable role in the DUV laser generation. For the long term, the search for new DUV NLO crystals has focused on the alkali‐metal inorganic acid salts where the variations of anionic groups are considered. Herein, another design strategy is reported, in which the alkali‐metal cations are substituted by the asymmetrical non‐π‐conjugated cations to harmonize the above mutual restrictions. Accordingly, LiN 2 H 5 SO 4 (LNHSO) crystal is obtained by the substitution of [N 2 H 5 ] + for alkali‐metal ions in LiMSO 4 (M = Na, K, and Rb). Compared with LiMSO 4 (M = Na, K, and Rb), LNHSO exhibits a moderate phase‐matching second‐harmonic generation response (0.6 × KH 2 PO 4 ), increased birefringence (two to three times that of other sulfates), and the widest bandgap (7.69 eV) among all the reported NLO sulfates. The structural analysis and theoretical calculation reveal that the structural anisotropy, arrangement, and packing configuration of the novel NLO‐active [N 2 H 5 ] + cation in LNHSO effectively promotes the optical properties and enlarges the bandgap. This work provides an innovative perspective for the rational design and synthesis of DUV NLO materials with excellent performance.