Non-centrosymmetric Selenides AZn₄ In₅ Se₁₂ (A=Rb, Cs): Synthesis, Characterization and Nonlinear Optical Properties

Two new non-centrosymmetric polar quaternary selenides, namely, RbZn₄ In₅ Se₁₂ and CsZn₄ In₅ Se₁₂ , have been synthesized and structurally characterized. They exhibit a 3D diamond-like framework (DLF) consisting of corner-shared MSe₄ (M=Zn/In) tetrahedra, in which the A⁺ ions are located. Both compounds are thermally stable up to 1300 K and exhibit large transmittance in the infrared region (0.65-25 μm) with measured optical band gaps of 2.06 eV for RbZn₄ In₅ Se₁₂ and 2.11 eV for CsZn₄ In₅ Se₁₂ . Inspiringly, they exhibit a good balance between strong second harmonic generation (SHG) efficiency (3.9 and 3.5×AgGaS₂ ) and high laser-induced damage thresholds (13.0×AgGaS₂ ). Theoretical calculations based on density functional theory (DFT) methods confirm that such strong SHG responses originate from the 3D DLF structure.