Point-to-volume engineering enables enhanced birefringence and a 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, a 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 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 the 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⁻ tetrahedral FBUs. This work provides an effective strategy for engineering hybrid halide materials with concurrently optimized linear and nonlinear optical properties.