Point-to-Volume Engineering Enables Enhanced Birefringence and Wide Bandgap in Hybrid Halide Ultraviolet Nonlinear Optical Crystals

Abstract

Ultraviolet nonlinear optical (UV NLO) crystals are important for advanced photonics, yet their development is hindered by the inherent trade-off among strong SHG response, wide bandgap and large birefringence. Herein, guided by systematic theoretical screening, the "two-in-one" flexible π-conjugated (C₄H₁₃N₅) ^2⁺ (MF) group was identified as a prospective functional building unit (FBU) owing to its superior polarizability anisotropy and hyperpolarizability. Its initial combination with Cl⁻ yielded C₄H₁₃N₅Cl₂ (MFC), which exhibits a wide band gap (4.64 eV) and a high SHG response (2.8 × KH₂PO₄ (KDP)), yet with a small birefringence of 0.02@546 nm. To address this limitation, we implemented a point-to-volume substitution strategy, replacing the discrete Cl⁻ anions in MFC with distorted [ZnCl₄] ^2-tetrahedra, yielding a novel zero-dimensional (0D) organic-inorganic hybrid halide (C₄H₁₃N₅)ZnCl₄ (MFZC). This structural evolution simultaneously enhances most of key optical properties: bandgap widening to 4.72 eV, birefringence enhancement to 0.09 at 546 nm, and retention of a strong SHG response of 2.2 × KDP. Theoretical and structural analyses indicate that the improved properties originate from the synergistic alignment of organic MF cations and inorganic [ZnCl₄] ^2-tetrahedra FBU. This work provides an effective strategy for engineering hybrid halide materials with concurrently optimized linear and nonlinear optical properties.