Noncentrosymmetric (KBa0.5)In2Se4 and (K0.58Ba0.71)(Ga0.89In1.11)Se4 derived from centrosymmetric BaIn2Se4 via partial cation substitution: structural chemistry and nonlinear optical activity

Two new noncentrosymmetric multinary selenides, (KBa0.5)In2Se4 (1) and (K0.58Ba0.71)(Ga0.89In1.11)Se4 (2), have been synthesized using a high-temperature flux method at 1123 K. They crystallize in the tetragonal noncentrosymmetric space group I4cm with cell parameters: a = 8.0277(2), c = 6.7654(3) Å, V = 435.99(3) Å3 for 1; a = 8.0296(4), c = 6.5403(9) Å, V = 421.68(7) Å3 for 2 vs the orthorhombic centrosymmetric space group Fddd for BaIn2Se4. Their one-dimensional structures are constructed by [(MSe2)–]n (M = Ga, In) chains composed of MSe4 tetrahedra via sharing edges, and K+/Ba2+ cation occupying the interchain cavities. They both show moderate second-harmonic generation responses about 0.5 × AgGaS2@2.1 μm. UV-vis-NIR diffuse reflectance spectroscopy measurement shows that the optical band gaps of 1 and 2 are about 2.60 and 2.34 eV, respectively. Theoretical calculations are also carried out to analyze their electronic structures and optical properties.