Explorations of New SHG Materials in the Alkali-Metal–Nb5+–Selenite System

Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn–Teller (SOJT) distorted Nb5+ cation, namely, Na2Nb4O7(SeO3)4 (P1̅; 1) and NaNbO­(SeO3)2 (Cmc21; 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4]2– anionic network composed of 2D [Nb4O11(SeO3)2]6– layers which are further bridged by additional SeO32– anions via corner sharing; the 2D [Nb4O11(SeO3)2]6– layer is formed by unusual quadruple [Nb4O17]14– niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb–O–Se bridges. The polar compound 2 features a 1D [NbO­(SeO3)2]− anionic chain in which two neighboring Nb5+ cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal–Nb5+–Se4+/metal–Nb5+–Te4+–O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.