Halide anion-mediated crystal structure engineering in guanidinoacetic acid halides for tunable birefringence

Abstract

Birefringent crystals for the solar-blind ultraviolet region are essential for advanced optoelectronic applications. This work demonstrates the use of halide anions (F⁻, Cl⁻, Br⁻, I⁻, and I₃⁻) as structural regulators to tune optical anisotropy in a series of new guanidinoacetic acid (GAA)-based halide crystals. Single-crystal XRD reveals that the chloride and bromide derivatives crystallize in non-centrosymmetric space groups, while the fluoride, iodide, and polyiodide analogues are centrosymmetric. The solar-blind ultraviolet-transparent chloride and bromide compounds exhibit moderate birefringence (Δn = 0.154 and 0.022 at 546 nm). In contrast, the centrosymmetric fluoride, iodide, and polyiodide crystals show exceptionally high birefringence (Δn = 0.227, 0.290, and 0.530 at 546 nm). The systematic variations in packing motifs and interplanar dihedral angles among GAA⁺ cations govern the overall crystal symmetry; concurrently, birefringence is precisely tunable via the selection of distinct halide anions. This study presents a rational crystal engineering strategy for designing birefringent materials with tailored optical performance.