Controlling d¹⁰ Metal in Phenylthiourea‐Based Hybrid Metal Halides to Actualize Dimensional Transformation and Significantly Improve the Birefringence

Abstract The development of high‐performance birefringent crystals is crucial for optoelectronic functional materials. However, the current commercial crystals generally have a birefringence below 0.3, making it difficult to meet high standards requirements. Therefore, designing and synthesizing new crystals with high birefringent performance has become a focus of research. Combining distorted metal cation polyhedra with π‐conjugated organic groups is an effective strategy for obtaining excellent birefringent crystals. At the same time, well‐oriented anisotropic units and low‐dimensional structures are also significant. In this work, a new phenylthiourea‐based hybrid metal halide (C₇H₈N₂S)₄CdCl₂, is synthesized which shows 0D (zero‐dimensional) structure and a relatively large birefringence (Δn = 0.17@546 nm). However, the birefringence value is below 0.3. Hence, dimension regulation strategy is conducted, and successfully synthesized a novel Hg‐based hybrid metal halide (C₇H₈N₂S)HgCl₂. (C₇H₈N₂S)HgCl₂ exhibits 1D(one‐dimensional) structure composed of special [(C₇H₈N₂S)HgCl₂]_∞ chains. Notably, it is an excellent experimental birefringence (Δn = 0.49@546 nm), which is much elevated and stands as the highest among chalcohalides, surpassing all inorganic halides. Structural analysis and theoretical calculations show that the interaction between C₇H₈N₂ and HgCl₂S or CdCl₂S₄ plays a crucial role in optical anisotropy. This study demonstrates that dimension regulation is an effective strategy for developing high‐performance birefringent materials.