Alkaline-Alkaline Earth Fluoride Carbonate Crystals ABCO₃F (A = K, Rb, Cs; B = Ca, Sr, Ba) as Nonlinear Optical Materials

A new series of alkaline–alkaline earth fluoride carbonates (KSrCO3F, RbSrCO3F, KCaCO3F, RbCaCO3F, CsCaCO3F, and Cs3Ba4(CO3)3F5) were synthesized by spontaneous crystallization with molten fluxes. Their crystal structures, except for Cs3Ba4(CO3)3F5, exhibit the stacking of [AF]∞ (A = K, Rb, Cs) and [B(CO3)]∞ (B = Ca, Sr) layers, and the coplanar alignment of [CO3] triangles. The results from the UV–vis diffuse reflectance spectroscopy study of the powder samples indicated that the short-wavelength absorption edges were all below 200 nm, except for Cs3Ba4(CO3)3F5, which is about 210 nm. Second-harmonic generation (SHG) on polycrystalline samples was measured using the Kurtz and Perry technique, which indicated that these carbonates are all phase-matchable materials in both visible and the UV region, and their measured SHG coefficients were about 3.33, 3.33, 3.61, 1.11, 1.11, and 1.20 times as large as that of d36 (KDP), respectively.本研究通过熔融助熔剂自发结晶法，合成了一系列新型碱金属-碱土金属氟化物碳酸盐，包括KSrCO3F、RbSrCO3F、KCaCO3F、RbCaCO3F、CsCaCO3F及Cs3Ba4(CO3)3F5。除Cs3Ba4(CO3)3F5外，其余化合物的晶体结构均呈现[AF]∞（A=K、Rb、Cs）与[B(CO3)]∞（B=Ca、Sr）层状堆叠，且[CO3]三角形呈现共面排布。对粉末样品开展紫外-可见漫反射光谱测试，结果显示：除Cs3Ba4(CO3)3F5的短波吸收边约为210 nm外，其余化合物的短波吸收边均低于200 nm。采用库尔茨-佩里测试法对多晶样品开展二次谐波产生（Second-harmonic generation, SHG）表征，结果表明：这类碳酸盐在可见光及紫外波段均为相位匹配材料，其实测SHG系数分别约为磷酸二氢钾（KDP）d36系数的3.33、3.33、3.61、1.11、1.11及1.20倍。