Five-Membered [C<sub>5</sub>O<sub>5</sub>]<sup>2<b>–</b></sup> Rings as Birefringence-Active Genes for Ultrabirefringent Crystals
Jiachen Lu, Yang Li, Kang Min Ok
Abstract
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 [C 5 O 5 ] 2–, and demonstrate its potential by synthesizing four alkali metal croconate crystals: Na 2 C 5 O 5 ·2H 2 O ( NNCO·2H 2 O ), Na 2 C 5 O 5 ·3H 2 O ( NNCO·3H 2 O ), NaCsC 5 O 5 ( NCCO ), and NaRbC 5 O 5 ( NRCO ). Remarkably, NNCO·2H 2 O and NNCO·3H 2 O exhibit ultrahigh experimental birefringence values of Δ n exp(010) = 1.062 and Δ n exp(01̅1) = 0.893 at 546 nm, respectively. These exceptional values arise from the highly anisotropic nature and well-aligned arrangement of the [C 5 O 5 ] 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 [C 5 O 5 ] 2– unit is the primary contributor to the observed optical anisotropy. Notably, NNCO·2H 2 O, NNCO·3H 2 O, 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 mm 3 . This work establishes [C 5 O 5 ] 2– as a new and effective structural motif for designing advanced birefringent materials, offering both outstanding optical performance and practical crystal growth capabilities.