Breaking Layered Limitations via Electrostatic-to-Covalent Conversion toward Three-Dimensional T2-Cluster-Based Chalcogenides

The inherently limited structural connectivity and anisotropic growth characteristics of two-dimensional (2D) layered nonlinear optical (NLO) crystals severely constrain their crystal growth potential, rendering the suppression of layering tendencies imperative. Our previous work employed a polycation-substitution strategy to transform centrosymmetric RbGaS 2 into noncentrosymmetric [ABa 2 Cl][Ga 4 S 8 ] (A = Rb, Cs), yet the 2D T2-supertetrahedral architecture was retained due to persistent electrostatic interlayer interactions. Herein, through Ba-to-Mg compositional replacement together with adjustments to the synthesis conditions, we successfully obtained two new 3D salt-inclusion chalcogenides, A[A 4 Mg 3 Cl 3 ][Ga 12 S 22 ] (A = Rb, Cs). During this process, the original 2D layered architecture is reorganized into a fully interconnected 3D framework, accompanied by an electrostatic-to-covalent conversion of interlayer interactions, namely, from predominantly Ba–S/Ba–Cl electrostatic interactions to more covalent Mg–S/Mg–Cl interactions. Theoretical calculations further support the presence of covalent contributions in the Mg–S/Mg–Cl interactions. Remarkably, both compounds not only overcome the inherent limitations of layered configurations but also maintain promising NLO functionality, including wide band gaps (3.47–3.54 eV) that contribute to high laser-induced damage thresholds (10.4 and 9.8 × AgGaS 2 ), along with competitive second-harmonic generation responses (1.2–1.7 × AgGaS 2 ). These results provide a useful structural design perspective for developing 3D T2-cluster-based chalcogenides with promising NLO properties.