Two Rare Earth-Based Sulfates with Enhanced Birefringence and Broad Optical Transparency Window

Abstract

Discovering sulfate optical crystals with enhanced birefringence and a broad transparency window is of great interest but remains challenging. Herein, two new rare-earth sulfate crystals NaNH₄Y₂(SO₄)₄ and NaKY₂(SO₄)₄ were designed and successfully synthesized by introduction of rare-earth Y³⁺ ions into alkali metal sulfates. Transparent millimeter-sized single crystals were grown by a facile hydrothermal method, with NaNH₄Y₂(SO₄)₄ reaching dimensions up to 4 × 4 × 2 mm³. Both compounds are isostructural and built from identical structural units, namely [YO₈] dodecahedra and [SO₄] tetrahedra. They exhibit enhanced birefringence (0.040−0.044 at 550 nm) and a broad optical transmittance range with absorption cut-off edges below 200 nm. This work provides a valuable pathway for developing sulfate optical crystals that combine broad optical transparency with enhanced birefringence.