Na₂CdGe₂Q₆ (Q = S, Se): two metal-mixed chalcogenides with phase-matching abilities and large second-harmonic...

In view of their inherent defects for commercial infrared nonlinear optical (IR NLO) materials, exploration for new IR NLO materials with excellent performance is an imperative and meaningful work. Herein, we report the successful design and synthesis of two metal-mixed chalcogenides containing divalent cations with d¹⁰ electronic configuration: Na₂CdGe₂S₆ and Na₂CdGe₂Se₆. Both of them crystallize in the polar Cc space group and exhibit three-dimensional tunnel structures constructed by the CdQ₄ tetrahedra and _∞[GeQ₃]_n chains with Na⁺ located in the tunnels. Furthermore, an interesting space group transformation between the monoclinic (Cc) and tetragonal (I4/mcm) systems from Na₂CdGe₂Se₆ to Na₂ZnGe₂Se₆ was discovered, which may arise from the different connection types of their building blocks (more flexible corner-sharing type in Cc while tightly edge-sharing type in I4/mcm). Remarkably, they exhibit type-I phase-matching abilities and large second harmonic generation (SHG) responses (0.8 and 2 times that of benchmark AgGaS₂ at 2.09 μm fundamental light). Notably, Na₂CdGe₂S₆ satisfies the essential requirements (coexistence of large NLO response and high laser damage threshold) as one excellent IR NLO material. The structure-property relationship has also been investigated through theoretical calculations and the results indicate that the origin of their NLO effects can be attributed to CdQ₄ and GeQ₄ units.