Enhancement of Second-Order Optical Nonlinearity in a Lutetium Selenite by Monodentate Anion Partial Substitution

The systematic modification of the molecular structure of nonlinear optical (NLO)-active materials is a very attractive approach to the improvement of NLO performance. In this study, one lutetium selenite compound Lu(SeO3)(HSeO3)(H2O)·(H2O) (1) was synthesized by hydrothermal means, while a further two lutetium selenites Lu3F(SeO3)4 (2) and Lu(SeO3)(NO3)(H2O) (3) were successfully synthesized by anion partial substitution (APS) of the parent structure 1 under hydrothermal conditions. Specifically, substitution of the [HSeO3]– anions in the noncentrosymmetric 1 by F– or [NO3]− anions with differing denticity leads to the formation of polar 2 and centrosymmetric 3. Our study reveals that the denticity of the secondary functional anions has a significant influence on the coordination environments of the rare-earth-metal cation Lu3+ and consequently the molecular configuration and NLO performance of the resultant multidimensional selenites. In contrast to 1, which displays a weak second-harmonic generation (SHG) response (0.1 × KH2PO4 (KDP)), 2 exhibits a greatly improved NLO performance, including a strong SHG signal (2.5 × KDP, the highest value among rare-earth-metal-based NLO selenites), a wide band gap (3.57 eV) and optical transparency window (0.35–10.3 μm), high thermal stability (∼550 °C), and a large laser damage threshold (36 × AgGaS2). These results suggest that 2, as the first example of a fluorinated lutetium selenite, is a strong NLO candidate crystal spanning a region from the near-ultraviolet to the mid-infrared. These APS studies highlight a new feasible approach toward high-performance NLO crystals.