Polar Lithium l-Aspartate Monohydrate: A Semiorganic Ultraviolet Transparent Nonlinear Optical Crystal

Ultraviolet (UV) nonlinear optical (NLO) crystals capable of expanding common laser sources to the UV regime are core components of solid-state lasers. Currently, exploring UV NLO crystals in hybrid organic-inorganic systems is receiving increasing attention. Herein, we report the synthesis, characterization, and linear and NLO properties of a semiorganic UV NLO crystal, namely, lithium l-aspartate monohydrate, Li(L-OOCNH3CHCH2COO)·H2O (1). The title compound crystallizes in the noncentrosymmetric polar monoclinic space group P21. It exhibits layered structures composed of [LiO4] tetrahedra, l-aspartate, and isolated H2O molecules; the layers are further strengthened by robust interlayer N-H···O hydrogen bonding to form the whole architectures. Optical measurements revealed that compound 1 exhibits a phase-matchable second-harmonic generation (SHG) response with powder efficiency 0.9 times that of KDP, a short UV transmission cutoff edge at 208 nm, and wide optical bandgap of 5.42 eV. Frist-principles calculations were conducted to elucidate the origin of the underlying linear and NLO properties.