Eu 2 MGe 2 OS 6 (M = Mn, Fe, Co): Three Melilite-Type Rare-Earth Oxythiogermanates Exhibiting Balanced Nonlinear-Optical Behaviors

Metal oxychalcogenides as candidates for novel mid-infrared nonlinear-optical materials have attracted great interest due to the distinctive advantages of oxides and chalcogenides in this field. Herein, the first melilite-type rare-earth (RE) oxythiogermanates Eu 2 MGe 2 OS 6 [M = Mn ( 1 ), Fe ( 2 ), Co ( 3 )] are obtained by combining RE metals with localized f electrons, magnetic transition metals with delocalized d electrons, and the highly distorted mixed anionic group [GeOS 3 ] into one structure. They belong to the tetragonal P 4̅2 1 m space group, and highly distorted [EuOS 7 ] bicapped trigonal prisms bridge adjacent {[MGe 2 OS 6 ] 4– } ∞ layers to build the three-dimensional network. Their optical band gaps are determined as 2.40, 2.11 and 2.14 eV, and they show moderate second-harmonic-generation (SHG) responses (0.3, 0.3 and 0.5 × AGS) and large laser-induced damage thresholds (2.77–8.31 × AGS). Theoretical calculation results indicate that the synergistic effect of [EuOS 7 ] and [MS 4 ] units acts on the SHG effect. This work enriches the crystal chemistry of melilite-structure materials.