Salt-Inclusion Rare-Earth Chalcogenides: Exploring Infrared Nonlinear Optical Crystals via a Guest-Framework...

Developing high-performance mid-infrared nonlinear optical (NLO) crystals is crucial for modern laser technology. Here, three novel salt-inclusion Eu-based chalcogenides (SICs) were successfully synthesized via a guest-framework modulation strategy: [KEu₄Cl][Ge₃S₁₀] (1), [Eu₄Cl₂][Ge₃S₉] (2), and [Eu₇Cl₂][Ge₆Se₁₈] (3). They exhibit diverse structures, where the [Ge₂Se₆]^4- dimer violates Pauling's third rule. 1-3 exhibit SHG responses of 0.3-0.9 × AGS and suitable bandgaps of 1.88-2.15 eV. Notably, 3 is the first Ge-containing SIC to demonstrate phase-matching behavior. Theoretical calculations reveal that the [Ge₂Se₆]^4- dimer in 3 exhibits enhanced polarizability anisotropy and hyperpolarizability due to its unique connectivity mode. This work systematically demonstrates the potential of rare-earth elements in expanding the structural diversity of SICs and optimizing their NLO properties.