Structural Modulation of Anionic Group Architectures by Cations to Optimize SHG Effects: A Facile Route to New NLO Materials in the ATCO3F (A = K, Rb; T = Zn, Cd) Series

A new series of alkali-transition metal fluoride carbonates (KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F) have been synthesized under subcritical hydrothermal conditions. All crystals are isostructural with the acentric space group P6̅c2 (188). They were structurally characterized by X-ray single crystal diffraction and exhibited the stacking of alternating AF∞(A = K, Rb) and TCO3∞(T = Zn, Cd) layers connecting adjacent layers by infinite T–F–T (T = Zn, Cd) chains parallel to c-axis. We found that all TCO3(T = Zn, Cd) building units aligned perfectly parallel in any given layer, but the rotation from one layer to the next resulted in the nonparallel arrangement of CO3 groups between two adjacent TCO3∞ (T = Zn, Cd) layers. In this work, the relative rotation of CO3 groups between two successive layers was successfully controlled by introducing cations of different sizes into the structures, which led to different relative rotation angles of CO3 groups, yielding varying second harmonic generation (SHG) effects for each fluoride carbonates. The SHG measurement indicates these compounds are all phase-matchable materials in both the visible and the UV region, and the experimental SHG responses are approximately 4.58, 2.84, 1.76, and 0.83 times that of KH2PO4 (KDP) for KCdCO3F, RbCdCO3F, KZnCO3F, and RbZnCO3F, respectively. All new compounds exhibit wide transparent regions ranging from the UV to the near IR, which suggest that they are promising UV NLO materials. In addition, the differences of the structures and NLO properties of A1+M2+CO3F-type crystals were summarized, and their structural design ideas and methods with respect to the structural modulation of anionic group architectures by cations to optimize SHG effects were detailed.