C(NH₂)₃MoO₃(IO₃): A Molybdenyl Iodate with Giant Birefringence Designed via a Cation–Anion Synergetic Interaction Strategy

Birefringent crystals are extensively utilized across diverse optical applications due to their unique property of splitting incident light into dual refracted rays, thereby modulating and controlling light polarization. The pursuit of promoting the birefringence of such crystals to facilitate device miniaturization has recently emerged as a prominent area of focus. In this investigation, we introduce two molybdenyl iodates, namely, C(NH₂)₃MoO₃(IO₃) and Rb₂MoO₂(I₂O₆)(IO₃)₂, conceived through a “cation–anion synergetic interaction” strategy. Each compound exhibits a one-dimensional chain structure. Despite processing similar wide band gaps (3.33 and 3.22 eV), these materials display a variance in their birefringence (Δn = 0.426 and 0.261 @546 nm). Notably, C(NH₂)₃MoO₃(IO₃) showcases the highest birefringence among all hitherto reported molybdenyl iodates, signifying its potential as a high-performance birefringent crystal. Theoretical analyses indicate that the C(NH₂)₃⁺ cation, acting as a birefringence-active unit, significantly bolsters the birefringence of molybdate iodates. Moreover, the presence of extensive hydrogen-bonding interactions between C(NH₂)₃⁺ cations and iodates influences the orientation of the highly anisotropic iodates, thereby further enhancing the birefringence of C(NH₂)₃MoO₃(IO₃). This research paves the way for the future exploration of organic–inorganic hybrid molybdenyl iodates exhibiting exceptional optical performance.