Na<sub>8</sub>Lu<sub>2</sub>(CO<sub>3</sub>)<sub>6</sub>F<sub>2</sub> and Na<sub>3</sub>Lu(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>: Rare Earth Fluoride Carbonates as Deep-UV Nonlinear Optical Materials

Two nonlinear optical fluoride carbonate crystals (Na<sub>8</sub>Lu<sub>2</sub>(CO<sub>3</sub>)<sub>6</sub>F<sub>2</sub> and Na<sub>3</sub>Lu­(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>) have been synthesized under subcritical hydrothermal condition. Both crystals crystallize in the noncentrosymmetric space group <i>Cc</i> (No. 9). The structure of Na<sub>8</sub>Lu<sub>2</sub>(CO<sub>3</sub>)<sub>6</sub>F<sub>2</sub> with high density of [CO<sub>3</sub>] groups is described as 1D [Na<sub>5</sub>Lu­(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>] chains connected by [CO<sub>3</sub>] triangles, forming an intricate three-dimensional framework. Lu<sup>3+</sup> and Na<sup>+</sup> cations are alternatively ordered and disordered in the cavities of the 3D network. The structure of Na<sub>3</sub>Lu­(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub> is built up from the [NaLu­(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub>] layers which are separated by other Na<sup>+</sup> cations. The [CO<sub>3</sub>] anionic groups arrange approximately coparallel to the plane. The second harmonic generation (SHG) measurement indicates that Na<sub>8</sub>Lu<sub>2</sub>(CO<sub>3</sub>)<sub>6</sub>F<sub>2</sub> and Na<sub>3</sub>Lu­(CO<sub>3</sub>)<sub>2</sub>F<sub>2</sub> have large SHG responses that are approximately 4.29 and 4.21 times KH<sub>2</sub>PO<sub>4</sub> (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.