EuMn6Ga6S16: A Wide Bandgap Europium-Based Nonlinear Optical Sulfide

Abstract 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.