EuMn6Ga6S16: A Wide Bandgap Europium-Based Nonlinear Optical Sulfide.

Transition-metal chalcogenides have emerged as promising infrared nonlinear optical (NLO) materials due to their favorable d-s, d-p, and d-f transitions that can enhance NLO response. However, the concomitant d-d transitions in transition-metal centers often induce undesired bandgap narrowing, severely limiting their practical applications. Herein, a new Eu-based transition-metal sulfide, EuMn6Ga6S16 (EMGS), has been successfully synthesized by a high-temperature solid-state reaction. Notably, the [MnS6] units effectively suppress the undesirable d-d transitions while preserving the beneficial d-p transitions for NLO activity, thereby mitigating the bandgap-narrowing effect commonly observed in transition-metal chalcogenides. Consequently, EMGS exhibits a phase-matchable second-harmonic generation response and the widest bandgap (2.82 eV) among Eu-based chalcogenide NLO materials. Theoretical calculations further reveal that the bandgap is predominantly governed by the synergistic contribution of both [MnS6] and [GaS4] units, whereas the NLO performance is mainly attributed to the [MnS6] units. This work not only provides a viable strategy for mitigating the adverse effects of d-d transitions through rational structural design but also expands the family of Eu-based chalcogenides for infrared NLO applications, paving the way for the development of new NLO materials with a wide bandgap.