Na₈Lu₂(CO₃)₆F₂ and Na₃Lu(CO₃)₂F₂: Rare Earth Fluoride Carbonates as Deep-UV Nonlinear Optical Materials

Two nonlinear optical fluoride carbonate crystals (Na 8 Lu 2 (CO 3 ) 6 F 2 and Na 3 Lu(CO 3 ) 2 F 2 ) have been synthesized under subcritical hydrothermal condition. Both crystals crystallize in the noncentrosymmetric space group Cc (No. 9). The structure of Na 8 Lu 2 (CO 3 ) 6 F 2 with high density of [CO 3 ] groups is described as 1D [Na 5 Lu(CO 3 ) 2 F 2 ] chains connected by [CO 3 ] triangles, forming an intricate three-dimensional framework. Lu 3+ and Na + cations are alternatively ordered and disordered in the cavities of the 3D network. The structure of Na 3 Lu(CO 3 ) 2 F 2 is built up from the [NaLu(CO 3 ) 2 F 2 ] layers which are separated by other Na + cations. The [CO 3 ] anionic groups arrange approximately coparallel to the plane. The second harmonic generation (SHG) measurement indicates that Na 8 Lu 2 (CO 3 ) 6 F 2 and Na 3 Lu(CO 3 ) 2 F 2 have large SHG responses that are approximately 4.29 and 4.21 times KH 2 PO 4 (KDP), respectively. The responses are also phase-matchable in the visible region. In addition, it exhibits wide transparent regions ranging from UV to near IR with a short UV cutoff edge (<200 nm), which suggests that the new compounds are promising as deep-UV NLO materials.