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 SeO₃^2– 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.