Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent Li₂Na₂B₂O₅ Material with an Unvalued B₂O₅ Functional Gene

Birefringent materials, the key components in modulating the polarization of light, are of great importance in optical communication and the laser industry. Limited by their transparency range, few birefringent materials can be practically used in the deep ultraviolet (DUV, λ < 200 nm) region. Different from the traditional BO₃- or B₃O₆-based DUV birefringent crystals, we propose a new functional gene, the B₂O₅ unit, for designing birefringent materials. Excitingly, the synergistic combination of Li₄B₂O₅ and Na₄B₂O₅ generates a new compound, Li₂Na₂B₂O₅, with enhanced optical properties. The Li₂Na₂B₂O₅ crystal with a size of up to 35 × 15 × 5 mm³ was grown by the top-seeded solution growth (TSSG) method, and its physicochemical properties were systematically characterized. Li₂Na₂B₂O₅ features a large amount of birefringence (0.095@532 nm), a short DUV cutoff edge (181 nm) with a high laser-induced damage threshold (LDT, 7.5 GW/cm² @1064 nm, 10 ns), favorable anisotropic thermal expansion (α_a/α_b = 5.6), and the lowest crystal growth temperature (<609 °C) among the commercial birefringent crystals. Moreover, the influences of the B₂O₅ structural configurations on the optical anisotropy were explored. The fascinating experimental results will provide a prominent DUV birefringent crystal and an effective synthesis strategy, which can facilitate the design of DUV birefringent materials.