Dimensionality Engineering of Fluorozirconium Phosphates for Deep-UV-Transparent Crystals with Enhanced Second-Harmonic Generation and Birefringence

Phosphate crystals have attracted significant interest as platforms for deep-ultraviolet (deep-UV) optical materials, attributable to their rich structural chemistry and tunable optical properties. However, simultaneously achieving strong second-harmonic generation (SHG) and enhanced birefringence in phosphate systems remains a formidable challenge. Herein, through a dimensionality-engineering strategy, we report five deep-UV transparent fluorozirconium phosphate compounds, including the three-dimensional (3D) ZrPO₄F (I), two-dimensional (2D) Zr(H₂PO₄)(HPO₄)F·3H₂O (II) and (NH₄)₄Zr₄(HPO₄)₂(PO₄)₄F₄ (III), as well as the one-dimensional (1D) Rb2.9(NH₄)3.1Zr₄(H₂PO₄)F₂₁ (IV) and Rb4.2(NH₄)3.8Zr₄(PO₄)₂F₁₈ (V). These compounds exhibit clear dimension-dependent optical properties, with reduced dimensionality leading to enhanced SHG responses and larger birefringence. Notably, the fluorine-rich 1D compounds display the strongest SHG efficiency (up to 2.1 × KDP) and the largest birefringence (0.053 @ 550 nm) within the series. Structure-property analyses reveal that decreasing the structural dimensionality promotes increased local distortion of Zr-centered polyhedra and more uniform dipole alignment, thereby amplifying both SHG and birefringence. This work establishes dimensionality engineering of fluorozirconium phosphates as an effective strategy for fine-tuning key optical properties while maintaining deep-UV transparency.