Reverse Weak Polarity-Induced Ordered Layer Control for Enhanced Second-Harmonic Generation in Ultraviolet Nonlinear...

There is an urgent need to achieve a large second-harmonic generation (SHG) response in ultraviolet nonlinear optical (UV NLO) crystals to advance UV laser technologies. SHG efficiency depends on the alignment and properties of functional groups, particularly π-conjugated units with high hyperpolarizability. Layered structures are widely recognized as an optimal template for densely preorganizing π-conjugated units into orderly arrays, yet the interlayer linkers are typically metal cations that couple adjacent layers through largely non-directional ionic interactions, often producing random even antiparallel stacking that cancels macroscopic SHG. Herein, we propose a Reverse Weak Polarity-Induced Ordered-layer Control (RPIO) strategy, in which a "layer→linker→layer" polarity-transfer pathway-driven by dipole-dipole interactions-enforces consistent alignment of polar layers. Guided by this strategy, three isostructural compounds, RE(C₃H₂O₄)NO₃·4H₂O (RE = Y, Gd, Lu), are synthesized, featuring polar ${\left[\text{RE}\left(C_3{H}_2{O}_4\right){\left(H_{2}O\right)}_4\right]}_{\infty }^{+}$ layers coherently aligned induced by NO₃ ^- linkers. All compounds exhibit large SHG responses (8.5-9.5× KDP), wide band gaps (∼4.13 eV), sizable birefringences (∼0.135 at 589.3 nm), and favorable growth habits. By explicitly engineering interlayer connectivity, this work establishes a predictable and controllable route to coherently stack polar layers and thereby maximize SHG in layered UV NLO materials.