From CsKTaF7 to CsNaTaF7: Alkali Metal Cations Regulation to Generate SHG Activity**

Graphical Two new mixed-alkali metal tantalum fluorides CsATaF 7 (A=K, Na) were synthesized by hydrothermal reactions based on the parent compounds A 2 MF 7 (A=monovalent cations and M-d 0 transition-metal cation). Interesting structure transformation for β-K 2 TaF 7 , CsKTaF 7 , and CsNaTaF 7 are induced via alkali metal cation regulations. Among them, CsNaTaF 7 crystallizes in polar structure and own moderate SHG effect. Inorganic metal halides play important roles in wide range of areas including fluorescence, X-ray detection, and nonlinear-optics. Herein, two new mixed alkali metal tantalum fluorides, CsKTaF 7 and CsNaTaF 7 , have been obtained based on the strategy of cations regulation in A 2 MF 7 (A represents monovalent cations and M is d 0 transition-metal cation) system by a conventional hydrothermal route. CsKTaF 7 crystallizes in the centric Pnma space group, while CsNaTaF 7 crystallizes in the polar Cmc 2 1 space group and exhibits moderate and phase-matchable NLO activity. Both halides possess large optical band gaps above 5.0 eV. The crystal structure evolution, optical properties, and detailed theory calculations of these two halides were elucidated in this work.