Five-Membered [C₅O₅]^2- 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 (NNCO·2H₂O), Na₂C₅O₅·3H₂O (NNCO·3H₂O), NaCsC₅O₅ (NCCO), and NaRbC₅O₅ (NRCO). Remarkably, NNCO·2H₂O and NNCO·3H₂O exhibit ultrahigh experimental birefringence values of Δn_exp(010) = 1.062 and Δn_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 NCCO (2.24 eV) and NRCO (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, NNCO·2H₂O, NNCO·3H₂O, and NCCO readily form high-quality, centimeter-scale single crystals via a simple aqueous evaporation method, with NCCO 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.