Cs2Bi2OSi2O7: A Promising Bismuth Silicate Nonlinear Optical Crystal with Face-Sharing BiO5 Polyhedra Exhibiting Strengthened Second Harmonic Generation Response and Birefringence

Nonlinear optical (NLO) crystals have aroused burgeoning interest in recent years owing to their frequency conversion properties and versatile optoelectronic applications. Nevertheless, a current challenge is to excavate NLO crystals with superior overall performance, especially in configurationally rigid silicates. Herein, a new NLO bismuth silicate, Cs2Bi2OSi2O7 (CBSO), was successfully synthesized, which features a 3D structure containing rare face-sharing [BiO5] pyramids and [SiO4] groups. The introduction of highly polarized Bi3+ cations resulted in large induced polarization and an enhanced second harmonic generation (SHG) intensity of 3.5 KDP. Remarkably, CBSO fully meets the precedent conditions for NLO applications, including a wide transparent window, suitable birefringence (0.044 at 1064 nm), good thermal stability with congruent-melting behavior, and facile crystal growth. Besides, first-principles calculations were performed to elucidate the structure–property relationship of CBSO. These observations demonstrate that CBSO is a promising NLO material and provide new inspiration for designing novel SHG-active compounds in strongly covalent systems.