K₂Pb₃(CO₃)₃F₂ and KCdCO₃F: Novel Fluoride Carbonates with Layered and 3D Framework Structures

Two new mixed metal fluoride carbonates, KCdCO₃F and K₂Pb₃(CO₃)₃F₂, have been synthesized by solvothermal and solid-state techniques. KCdCO₃F crystallizes in the acentric nonpolar space group <i>P</i>6̅<i>m</i>2, and its structure features a three-dimensional anionic framework in which the CdCO₃ layers are further interconnected by bridging F^– anions with the negative charge balanced by K⁺ cations. K₂Pb₃(CO₃)₃F₂ crystallizes in the centrosymmetric space group <i>P</i>6₃/<i>mmc</i>, and its structure exhibits a layered anionic skeleton featuring corner-shared PbO₆F and PbO₆F₂ polyhedra. UV–vis diffuse reflectance spectroscopy studies show that the short-wavelength absorption edges of KCdCO₃F and K₂Pb₃(CO₃)₃F₂ are 227 and 287 nm, respectively. The second harmonic generation (SHG) measurement reveals that KCdCO₃F is a phase-matchable material for generation of doubled-frequency light at both 532 and 266 nm, with a large SHG response of approximately 5.2 times that of KH₂PO₄ (KDP) at 532 nm and a moderate SHG response of approximately 0.75 times that of β-BaB₂O₄ (BBO) at 266 nm. Therefore, it is a promising UV material for fourth harmonic generation on a 1064 nm Q-switched Nd:YAG laser