K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

Abstract

Cyanurate motifs are emerging functional basic blocks (FBBs) for nonlinear optical (NLO) materials and birefringent materials. Herein, we report two new potassium lead(II) hydroisocyanurates, namely, K₂Pb(H₂C₃N₃O₃)₄(H₂O)₄ (I) and K₃Pb(H₂C₃N₃O₃)₅(H₂O)₆ (II). I and II belong to acentric Cm and centric C2/m, respectively. Compound I features a novel 3D network formed by 2D [K₂PbO₈(H₂O)₄]¹²⁻ anionic layers interconnected by (H₂C₃N₃O₃)⁻ anions. The 2D [K₂PbO₈(H₂O)₄]¹²⁻ anionic layer is composed of 1D chains of edge-sharing K(1)O₆ and K(2)O₈ being further interconnected by 8-coordinated Pb²⁺ ions. Compound II exhibits a characteristic 3D framework based on 2D [K₃PbO₁₁(H₂O)₆]¹⁷⁻ anionic layers interlinked through (H₂C₃N₃O₃)⁻ groups, and the 2D [K₃PbO₁₁(H₂O)₆]¹⁷⁻ anionic layer is based on K–O/H₂O chains bridged by Pb²⁺ ions. Powder second harmonic generation (SHG) measurements of I revealed a moderate SHG response of approximately 2.6 × KDP with a type I phase-matching behavior under 1064 nm laser radiation. Theoretical calculations of I and II revealed their birefringence to be 0.325 and 0.193@532 nm, respectively. Their impressive optical properties mainly stem from stereochemically active lone pair (SCALP) Pb²⁺ cations and the planar π-conjugated (H₂C₃N₃O₃)⁻ groups.