Can Low Structural Anisotropy Produce High Optical Anisotropy? Anomalous Giant Optical Birefringent Effect in PI4AlI4 in Focus

Tetrahedral halides with broad transparency and large second harmonic effects have the potential to serve as mid-infrared wide-bandgap materials with balanced nonlinear-optical (NLO) properties. However, their regular tetrahedral motifs tend to exhibit low optical birefringence (Δn < 0.03) due to limited structural anisotropy, which constrains their practical phase-matched capability. A significant challenge in halide structural chemistry and material exploration is to enhance the Δn of tetrahedral halides while maintaining their balanced properties. The question of whether tetrahedra with low structural anisotropy can produce high optical anisotropy remains unanswered. In this study, in addition to the conventional strategy of enhancing Δn by increasing structural anisotropy, we identify a previously unreported strategy of enhancing Δn by increasing electronic anisotropy. This novel strategy model unprecedentedly enhances the Δn of an existing tetrahedral halide, PI₄AlI₄, to a degree not achievable by conventional techniques, with the value reaching up to 0.31@1 μm. The anomalous increase in Δn is achieved by anisotropic charge redistribution resulting from a unique charge transfer effect between the anionic and cationic tetrahedral motifs. First-principles analysis provides further corroboration of the detailed chemical bonding electronic distributions and charge transfer illustrations. It is predicted that analogous AsI₄AlI₄ and TeI₄ZnI₄ will exhibit a greater propensity for giant Δn (> 0.5@1 μm). This finding will greatly enrich the structural chemistry of sp³-hybridized tetrahedra and provide seminal ideas for the design and modulation of highly birefringent structures.