Reverse Weak Polarity‐Induced Ordered Layer Control for Enhanced Second‐Harmonic Generation in Ultraviolet Nonlinear Optical Crystals

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(C 3 H 2 O 4 )NO 3 ·4H 2 O (RE = Y, Gd, Lu), are synthesized, featuring polar layers coherently aligned induced by NO 3 − 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.