Five-Membered [C₅O₅]^2<b>–</b> Rings as Birefringence-Active Genes for Ultrabirefringent Crystals

Birefringent crystals are a vital class of optoelectronic materials capable of modulating and detecting the polarization state of light. The pursuit of high-performance birefringent materials has largely focused on incorporating functional units with strong polarization anisotropy and favorable alignment. In this work, we introduce a novel birefringence-active group (BAG), the five-membered ring [C₅O₅]^2-, and demonstrate its potential by synthesizing four alkali metal croconate crystals: Na₂C₅O₅·2H₂O (<b>NNCO·2H</b>_<b>2</b><b>O</b>), Na₂C₅O₅·3H₂O (<b>NNCO·3H</b>_<b>2</b><b>O</b>), NaCsC₅O₅ (<b>NCCO</b>), and NaRbC₅O₅ (<b>NRCO</b>). Remarkably, <b>NNCO·2H</b>_<b>2</b><b>O</b> and <b>NNCO·3H</b>_<b>2</b><b>O</b> exhibit ultrahigh experimental birefringence values of Δ<i>n</i>_exp(010) = 1.062 and Δ<i>n</i>_exp(01̅1) = 0.893 at 546 nm, respectively. These exceptional values arise from the highly anisotropic nature and well-aligned arrangement of the [C₅O₅]^2- units, combined with their moderate spatial density. In addition, the hydrated compounds feature wider bandgaps (2.67 and 2.55 eV) than their anhydrous analogues <b>NCCO</b> (2.24 eV) and <b>NRCO</b> (2.14 eV), thereby achieving a desirable balance between large birefringence and moderate bandgap─key criteria for optical applications. First-principles calculations and quantum chemical analysis confirm that the [C₅O₅]^2- unit is the primary contributor to the observed optical anisotropy. Notably, <b>NNCO·2H</b>_<b>2</b><b>O</b>, <b>NNCO·3H</b>_<b>2</b><b>O</b>, and <b>NCCO</b> readily form high-quality, centimeter-scale single crystals via a simple aqueous evaporation method, with <b>NCCO</b> reaching up to 10 × 9 × 2.5 mm³. This work establishes [C₅O₅]^2- as a new and effective structural motif for designing advanced birefringent materials, offering both outstanding optical performance and practical crystal growth capabilities.