Solvent-controlled syntheses of mixed-alkali-metal borates exhibiting UV nonlinear optical properties

The mixed-alkali-metal borates NaK(B₅O₈)(OH)·H₂O (1), NaK₆[(B₄O₅)(OH)₄]₃(OH)·C₂H₅OH (2) and Na_0.33K_1.67(B₄O₅)(OH)₄·3H₂O (3) have been solvothermally synthesized using various polar organic solvents. Compounds 1 and 2 crystallize in the centrosymmetric space groups P1 and R3c, respectively. The structure of 1 features a 2D-layered framework constructed by [B₅O₁₁]^7- primary building units in the ab plane, extending to a 3D framework linked by K⁺ and Na⁺ cations, while the structure of 2 can be described as isolated [B₄O₉]^6- primary building units connected by H-bonding interactions and K-O and Na-O bonds, forming a 3D supramolecular framework. Compound 3 crystallizes in the acentric space group P62c, and its UV nonlinear optical properties have been investigated for the first time. Second-harmonic generation (SHG) measurements show that 3 is type-I phase-matching, with a moderate SHG response ca. 0.94 times that of KH₂PO₄. The cut-off edge of 3 is 242 nm, which suggests that 3 is a potential UV NLO material. Density functional theory calculations have been employed on 3 to rationalize its band structure and electron density as well as the density of states.