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

Two nonlinear optical fluoride carbonate crystals (Na₈Lu₂(CO₃)₆F₂ and Na₃Lu­(CO₃)₂F₂) have been synthesized under subcritical hydrothermal condition. Both crystals crystallize in the noncentrosymmetric space group Cc (No. 9). The structure of Na₈Lu₂(CO₃)₆F₂ with high density of [CO₃] groups is described as 1D [Na₅Lu­(CO₃)₂F₂] chains connected by [CO₃] triangles, forming an intricate three-dimensional framework. Lu³⁺ and Na⁺ cations are alternatively ordered and disordered in the cavities of the 3D network. The structure of Na₃Lu­(CO₃)₂F₂ is built up from the [NaLu­(CO₃)₂F₂] layers which are separated by other Na⁺ cations. The [CO₃] anionic groups arrange approximately coparallel to the plane. The second harmonic generation (SHG) measurement indicates that Na₈Lu₂(CO₃)₆F₂ and Na₃Lu­(CO₃)₂F₂ have large SHG responses that are approximately 4.29 and 4.21 times KH₂PO₄ (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.